No, each mode should not be left to their own devices. If it turns out that the modes don't work well, that one is better than another, should they not be changed? For another example, pvp piracy is better than pve piracy, is it wrong to give pve piracy a boost?
Why should that be the case? If PP was/is suffering from a population problem, should FD not be allowed to make changes because that is what the community wants? I'd say no, they should find the underlying reasons why, and try to fix them. I think it should be the same in this case.
What would constitute a Mode 'not working well'? Who and by what criteria would that be assessed? If FD sees players using Private Groups more than open, what motivation would they have to try and shift the population? What about open should be preserved above Group? Who do FD decide to listen to, the fewer player's in open, or the larger population in any of the Groups? I can see no reason fro FD to favor open, by bribing players to use that mode. It makes no sense to eliminate choice, or subsidize one mode over another. Letting the player decide each time they log in, especially since that is the status quo, is the fairest way to design the game.
I don;t aquatint the Mode Question with general game content. They are vastly different issues. I think PP mechanics are more akin to modules, ships, and missions. A player expects buffs and nerfs to specific items in the game, and that content is ever changing. The modes are part of the basic character of the game. The current design has been part of the game virtually since it's inception. What is the underlying reason for open having population issues?
So, to my overall question; what makes open worth penalizing the other modes? Why shouldn't each player be able to enjoy the game in their own way, without being bribed or coerced into an environment not of their choosing?