Of course the FH2 developers think about moving to different games, but there are a large number of problems with switching over to a new game.
First of all, as you noted, moving to a game that isn't out yet brings with it some uncertainties. Yes, sure, if the game is a hit and if the developers decide to support mods it might be a good idea. But we won't know this until the game is released, which might be years from now, if it even gets released. And even if we suppose it gets released, is a hit and provides marvellous support for mods, then it will take years to port all our content (making new content is totally out of the question, there aren't enough developers) and on top of that, all our existing maps would be useless, so they'd have to be started from scratch, which again will also take years.
Moving to a new game is such a huge commitment, that there simply isn't room for any uncertainties.