At this point it sounds like there's no one left at the company who still cares.

That would be a death sentence, even if the company had good financials, which it also sounds like it does not.

Open sourcing it sounds very unlikely in that condition.

The only thing I can think of is to switch to a subscription model. If there's enough people who rely on it and need it to keep working despite Mac OS updates. Then it seems fair to continue paying for development?

(If there's enough people like that, maybe they could organize something and contact the company... maybe)