A) It doesn't need to be PMFs, it can be any faction. However, all of the biggest factions are player factions.
B)
DaVinci Corp. control 77 systems and are present in 111. On page 1 of eddb.io, listing by number of controlling systems, only 1 faction is not a PMF, the smallest on that first page controls 29 systems and is present in 56. The biggest non PMF is Alioth Indepdenents at 30 controlled systems, present in 77.
Now, i'm not sure what the cut off point should be, or whether it should be a "top X" which gain power status, what the criteria for rising, falling, or being replaced should be. But i think it is a fantastic "pipe dream", one which gives people in all modes an opportunity to participate in the fate of powers, and then PP can go on top, so when they become powers, it becomes more about direct player conflict, which is where the PvP can come into play. Powers need their champions maintaining their statuses after all!
Sure, its a bit of a pipe dream, but its a solution that might please a lot more than just making it open only, and it would give a mechanism for automated rising and collapsing.
However, not the only solution, it could be handled in other ways. After all, game companies hire game designers for a reason, i'm sure they could find a mechanism to make it work.
Of course, i don't expect anything like that to happen until the 2020 update is out of the way, FD clearly have their plans already laid out. But i'd rather they focus on doing something worthwhile with PP, even if it takes a lot of effort, rather than half-rear-end solution that only caters to a minority at the expense of others, and still doesn't solve the most fundamental problem with PP.