As I keep trying to explain (poorly, it seems) is that, according to Frontier, currently a significant majority of players play in Open. I sincerely doubt that this statistic changes appreciably among the Powerplay community. In fact, if Powerplay is truly more attractive to PvPers than it is to PvEers, as I've been told repeatedly, then this statistic should skew more in favor of Open vs the general community. Either way, it means that, with the exception of the hyper-competitive "win at all costs" type of player, the majority of players across all powers should be doing their activities Open, including hauling. If this isn't the case, then that means that Powerplay is more attractive to PvEers than it is to PvPers, in which case Powerplay going Open Only will be even a larger disaster than I suspect.
While the hyper-competitive types may have an outsize influence on all aspects of the game (including fortification), the current stalemate isn't caused by the modes. It's caused by the automatic fortification triggers. It's essentially an automatic win for the fortifier. This allows Powers to minimize the effort needed to maintain their their borders. This in turn means that once a system is fortified, any undermining beyond the trigger is likewise wasted, so the focus is on any systems that are the targets of an expansion.
This is great for the PvP players, because it puts a lot of players into a single system (or perhaps two), making it easy to find other players to fight. This is great for the "Centrists" due to the "needle in a haystack" effect. "Centrists" are much more willing to fly in Open, because they're not getting frustrated by frequent attacks, whether those attacks are successful or not. This is great for the PvE players, because they don't have to fly in Open at all. This isn't so great for the player-killers, though, because their "watering hole" is being dominated by PvPers, the PvEers are not playing in Open at all, and the "Centrists" are spread out among dozens of systems, and aren't as easy to kill as a PvEer. Which is why they don't bother with it at all, either directly via pledging to a Power, or most likely, as was seen in the first few weeks of Powerplay, unpledged murderers hanging out in capitals and control systems.
With the proposed changes to Powerplay, assuming mode agnosticism remains in place, the efforts of hyper-competitive players and non-Open players will mostly cancel each other out... although the rules still favor fortification efforts. Since fortification is no longer guaranteed, the Powers will need to contract, resulting in fewer systems overall. Things still haven't changed much for the PvPers, but with fewer systems overall, a wider selection of activities become possible... in theory, at least. Most likely, the "exciting" systems will remain the handful of expansions, which at least will be more frequent due to a much more dynamic Powerplay map.
Things still haven't changed for PvE players, because they still don't have to play in Open. The status quo will change for the "Centrists," since as the number of systems overall decreases, the number of attacks they experience increases. For some, this can result in a more exciting game. For others, it remains tolerable. And for some, the frequency of attacks becomes intolerable, and they switch to other modes.
The biggest change will be for player-killers, at least at first. At first, "Centrists" will become easier to find, as the Powers become more compact, and while they aren't as easy to kill as PvEers, at least they're easier to find now. But then the inevitable feedback loop happens, and as the least PvP inclined "Centrists" leave Open, player-killers once again find it hard to find someone to attack, and those that remain in Open become even harder to kill. Unable to get their "fix," the player-killers leave again.
Dropping the Open Only grenade, on the other hand, will have profound effects. The most immediate effects will be the PvE players simply quitting Powerplay altogether. This will result in a much greater contraction across all Powers than if the game had remained mode-agnostic. The hyper-competitive players, OTOH, will either quit, or start cheating, depending on how far they're willing to go to "win." The amount of combat logging will most certainly increase. "Centrists" will experience much greater attack frequencies due to the much smaller number of systems, and will start quitting Powerplay altogether, resulting in even greater shrinkage of the playing field, a feedback loop that will eventually result in the last of the "Centrists" quitting, and this is before we throw the player-killers into the mix, with the similar results to above.
At the end, the Powerplay map will consist mostly of the capitals, and perhaps a handful of other control systems, and only the PvPers will remain. Rather than being concentrated in a few systems, they'll be spread out all over the map, trying to keep things running. They might occasionally make raids, but mostly they will sit around and remissness about the glory days of old, when "vast fleets" of ships struggled against each other in epic combat... at least until the "next big thing" comes along.
Hopefully, the last one to leave will remember to turn off the stove and the lights.
I've seen this scenario play out far too many times to believe that Powerplay will be any different, especially when you factor in this game's networking architecture, and Frontier's lack of experience in multi-player games in general, and MMOs specifically. I'm still rather amazed that they managed to stumble onto a winning formula that has resulted in Open being played by a significant majority of players, though back before the Alpha, I was convinced this was due to learning from the mistakes of past games, rather than just blind luck.
I could be wrong. If Frontier decides to pull the Open Only grenade, I hope I am wrong, because there's no recovering from the effects if I'm right. Players who quit very rarely come back, and I'm far too fond of the potential of Powerplay (even if the execution was bad) to see it die off that way.
Until Powerplay goes Open only, it will be hard to know who it will attract and repel. The one thing that is clear to me though is Powerplay has to change and it has to be radical to give it purpose. This new re-purposing might indeed annoy some, but its a risk worth taking since the potential experience on offer would be epic.
I don't really get where you think powers will contract with mode agnostic Powerplay. Unless you blanket mega UM loads of systems you are still going to have an isolated faceless race between harvesting merits and hauling. Since you can't snipe this way any more, its just going to be a slog which will put more players off. Imagine that each cycle- it put you off even before this, imagine an unending grind that has no end and no enemy to face other than an ever rising indicator. The short is that its not adding anything to the game we don't already have.
While the fortification thresholds don't help, modes facilitate easy and dependable fortifying. As I outlined, if you have the tug of war fort idea in open it ratchets up the tension as more importance is put on each run. Without open, it simply makes for additional grind, amplifying the solo advantage and reducing the potency of that idea- its the hamster wheel gameplay that people hate.
Along with consolidation, fortification is far too strong. Most powers by now have decent BGS teams and have optimized systems. As it stands without open attenuating hauling, it will make no difference and make things worse. It also negates the skill element of actually intercepting and killing haulers who are most likely going to be in Cutters.
Plus, you forget even with rampant attacks and undermining, at a certain size Powers will have enough CC to do what they want. Sure, Powers will be smaller in Open only, but each expansion would mean more in gameplay and planning. It would also free space that FD have not added for the feature, allowing Powerplay to breathe and have more options for expansion as time goes on.