This division of exclusive PvP players and pve players in the powerplay community is entirely fabricated. As for numbers being reduced, that's your prediction. I don't predict that. I predict that many people who've been put off powerplay over the years would see it as another sign to return.
There are very PvP oriented players, there are people who enjoy both, there are PvEers who don't mind a bit of PvP and PvP encounters, and there are hardcore PvEers who want nothing to do with PvP.
We know open is the most popular mode, and i suspect a vast majority are people who spend their time focused on PvE, but don't mind the PvP when it happens.
I'm not fabricating any division. However, its clear, from the types of people who have posted over the years, some of those who push the hardest for open only are those who enjoy PvP at a level way above PvEers who don't mind a bit of PvP on the side. Its quite clear because those people wouldn't be pushing for this so hard, for them its a "if it happens it happens" kind of attitude, not something that pushes someone to be active on the forums about significant change. More or a take it or leave it attitude.
So, the division is more just categorizing along the lines of open only proponents vs those who are against, and probably a chunk of people who don't care about it enough to debate in the middle. However, i suspect a lot of those people enjoy the ability to switch mode when they want to, based on their mood and wouldn't appreciate it if FD took that option away.
As for your prediction, PP2 saw a massive upswell in the number of people signing up, including many people who had never done PP before or who had dismissed it. That kind of indicates that the reason for PPs popularity (or lack of it) is more to do with the mechanics/features than anything to do with modes.
Are there really that many people not doing PP specifically because of modes? Compare it with how many people do PP because they can enjoy it in PG/solo.
I very much doubt it.