I don't believe simple monetization would be an issue, personally. There are plenty of ways to drag some cash out of the fanchise (that we would consume with happy adoration). For starters, picture a T-Shirt with a smiley face next to a blocked 'g'. Or an actual leather backpack with the word 'MASTERWORK' down the sides. Or a Coffee Mug with "Unused by Dwarf" on it.
Either you'd get it or you wouldn't, and that's pretty trademark for the DF community from what I've witnessed.
So, I don't believe cash alone would stop the DF game as long as the Tarns were willing to work with a bit of that. As long as the donations can carry them though I can also see why they wouldn't want to get directly involved in things of that nature.
There will always be a hardcore fanbase to DF. Heck, even I come back off and on after a year of other stuff and learn new stuff and goof around. If DF was to die though, it will be when something similar enough comes out from a different indie that doesn't FPS to death or has a UI that isn't a mental blockade to get through. A large enough portion of the fanbase would move to that, enough that the Tarns would get discouraged. Personally I think that would be the only real killer, Toady One and Threetoes getting discouraged because of their fanbase leaving.
Barring that, as long as the Tarns are willing to keep refining and working on their dream game, it'll be here for us to keep dumping children into magma pits so they stop breaking our workshops and figuring out ways to weaponize dust breathing hydras... and trying to line their pits with pillows so they don't kill themselves while they're doing it.