But for example, Clare's Quest has a mechanism for fixing the compatibility of old saves with new versions of the game.
That generally only works if the game content hasn't so much changed as much as there's just stuff that gets added to it.
Generally, as long as you keep adding to the game, the save tends to keep working. But if you completely rework a section of a game and the save contains stuff from the old version of that, that's when it breaks.
The Claire's Quest thing states it checks to update plugins and scripts, which could hedge against a certain level of change probably, but likely not everything.
Even then ... that's still more the exception then the rule anyway. It's not like broken saves across versions is anything new after all.
It's not something you'd ever encounter on let's say a AAA game or even your average decent quality indie game.
Those games are generally made from scratch and they had to build their own save system. As such, they've got full control over what content goes into a save file, as well as full control to modify a save file upon update of the game or add in backwards compatibility when loading up a save from an old version.
RPGM doesn't really seem to offer that level of control.
Well That would be ok if this game wasnt updated once a year.
The technical side of things don't change just because you only release 1 version every year ya know.
Also, on account of the 'change tends to break it, adding generally does not' rule ... breaking saves is something that generally occurs on the early days of a game's (development) cycle.
Games I've kept an eye on generally have this problem when they're still figuring out where they're going. But once they've got a proper base, where they don't need to rework stuff too heavily anymore, then it's generally not something that happens very often anymore.
And I'm not talking about reworking some CG's either, that's just changing a picture asset after all.