Unless FDEV completely rewrites the networking stack and provides dedicated servers, this change wouldn't actually change anything. Currently the game is peer-to-peer for networking so you can easily block everyone in the world (except your friends) with simple Windows firewall rules (or router firewall rules) effectively turning Open into Solo or a Private Group (controlled by IP address).
So yeah, good luck with all that.
I suspect that if FD could detect people doing this it might be considered cheating. It depends on whether there could be a scenario where you might have strict firewall rules in place that just happened to also cause problems with matchmaking like you suggest.
However, there are probably other ways to cause matchmaking to fail that don't require you to mess with firewall rules. Perhaps changing your MTU to something silly could do that. I'm fairly certain that sucking all your bandwidth up with torrents could make it so that you don't get matchmaked, and if you did, then you would be lagging so bad in in relation to the other person they would be shooting at a place where you were not.
As I mentioned earlier, its a complitive mechanic, and competitive people will often do whatever it takes to "win". Furthermore, there might be those who are disgruntled with being forced into open but don't want to stop playing powerplay and feel that FD have betrayed them, so these people might also resort to doing things that would be frowned upon.
The question would be, what sort of percentage of players would we be talking about? If its single figures, its probably not much of an issue. If it is widespread, then it undermines the whole reason for open only.
This reminds me of one situation with PP as it is. I've seen accusations levelled of a certain power's supporters all playing in PG/solo. (naturally those making the accusations declare they are all in open... don't believe it for a second, but whatever). If that power's players are all doing it for efficiency/competitiveness, rather than they just prefer to play in PG/solo, then in an open only scenario they might just decide they will all continue to effectively play solo by using tricks to avoid matchmaking.
In short, a switch to open only might not do much to get more people playing in open (or being visible in open).
Its not a reason for FD not to make the change, but its another thing which might mean the result is not what people were hoping for.
Still, i remember when i was doing Powerplay, hauling away from A to B, over and over and over again (boring!!!!), and when i was in Open, i didn't get attacked once by a player. I have no idea why not. Was there no opposition? Or was it due to poor matchmaking? Maybe i'd just selected systems where the PvPers were not. I got lucky. Maybe the time of day when i was playing was a factor. Or maybe it was matchmaking. Somehow i never have problems matchmaking with my friends, they always appear. But i can fly all over alliance space and never see another player. I haven't seen a single random player in open in months while flying around my home system and surrounding space. But there must be players flying around in Open. Its very close to Alioth, a fairly popular area, especially with Powerplay players. Not seen a single Mahon supporter or, for that matter, anyone opposing Mahon either.
If, as claimed, there is a risk of being stopped by hungry PvPers looking to stop Mahon players doing stuff, where are they all? Or are powerplay's numbers so low that chances of meeting anyone are almost zero?
This then makes me wonder, even if FD made all the changes that these proposals are asking for, would there actually be so many more players playing as to make it likely to meet people? From what i've seen of the proposals, there isn't anything there that would get me playing Powerplay again. It would still remain the never ending boring grind that it is today.
Or are there actually loads of people flying around that i'm simply not being matchmaked with?