Like I said, the game itself is built on an increasingly shaky foundation (and, in fairness, they couldn't have known what a cluster it would become when they started the project 14 years ago—Inform is niche but it's still very well-established in the IF scene). Trying to get the game in its current state to even build sucks ass (last time I tried it, it took something like 12 minutes to build the game on a Ryzen 5900X—absolute madness), and that alone would dissuade anyone from contributing. It's completely understandable why the content well is drying up.
I can't really speak for their intentions on how they're handling their Patreon, of course. Looking at
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it's pretty clear that their numbers have been on a steady decline (and even when their earnings were public they were only doing okay-ish—like under $20k/year), and that certainly helped pick them up a bit. It definitely doesn't help that their rewards weren't exactly compelling. I mean, $25/mo for a rotating cheat code is obscene.
As far as making a new game is concerned, it's worth noting that Flexible Survival isn't the only "Flexible" game—two of them came before this. It wouldn't be unprecedented to create a fourth game in the loosely-connected series, though I imagine nowadays the sunk cost fallacy would kick in pretty hard.