I understand where people are coming from, not wanting to be forced into open to be targets for other players. I also understand PVPers wanting people forced into open for powerplay so they're not forced into hauling to do PP. I think that's really the crux of the dilemma: each side wants to stay where they are, but allowing both to do so eliminates an entire layer of powerplay activity.
If we force PP into open, haulers must be prepared to survive PVP somehow in order to keep affecting powerplay. Haulers don't seem to like this.
If we let haulers stay solo, PVPers MUST turn to hauling in order to affect powerplay at anywhere NEAR the same level because actively stopping haulers is now impossible. PVPers don't seem to like this.
Both decisions force both players into a single mode in order to affect powerplay, leaving Fdev with quite the conundrum if they want to make the feature actually work. I'm sure we all hope there's a hidden third answer somewhere--I know I do because I don't want to see half the playerbase leave, no matter WHICH side it is--but, for the sake of argument, if we assume we MUST choose one of the two above, making PP open only is actually better for the game for a simple reason: in order to affect powerplay, haulers will have to be more social, i.e. either fighting back against gankers or looking for protection from others. As powerplay is specifically described as a "player versus player" experience, this sounds like the more logical choice to me. Allowing solo players to have as much effect on PP from the immunity of solo play means that PVPers can't even exist. At best, they can gank NPC ships for merits, but this has nowhere near the impact of hauling 700+T of supplies per run; eventually they have to turn to hauling as well, defeating the game's philosophy of "blazing your own trail." That's what the open only crowd is getting at here: by letting powerplay happen in solo, haulers force pvpers to play their way.
I know this is a rather contentious point, but after a lot of consideration I really think open only PP just makes more sense in the biggest possible picture. It may require haulers to do more than just AFK haul from station to station, actually get involved with other people and take some real risks to support their power, but...cliche though it sounds and despite the memes, the game IS called "Elite Dangerous," not "Liberal Space Trucker 2018." Hard pill to swallow, but it's true.