To expand on this point, if you look at Linux development, multiple companies are contributing to the project! The product exists outside any single organisation.

Of course closed source products can also be collaborations, but open source + open development practices can make this a smooth and natural result.

If the DBMS's users wouldn't chip in enough money to cover the cost of developing it before the company went under, it seems hard to believe that they'll suddenly decide it's cost effective to pay for having it developed post-failure. If anything, it just implies that they've become so tightly coupled to the product that the DBMS vendor's failure represents a sudden existential crisis.

I think, more important that whether or not the DBMS is open source, just consider how critical the DB is. If you can't live without it, stick with a DB that you can be virtually 100% sure isn't going anywhere soon. Save your experimentation with new products for the small and less-important stuff, and make sure the small stuff doesn't reach some critical mass before the DB vendor does.

Someone? You shouldn't pick an opensource project hoping someone motivated picks it up soon after it's abandoned. The primary company going down is usually it, especially when the project hasn't even picked up enough community yet.

If open source only works when you pay for it, it's just an abstraction layer around ordinary commercial software, with the same pressures. In fact, most of the people qualified to fix Linux video drivers are already employed full-time at a graphics card manufacturer (with its sales pressures) or a commercial redistributor (with its sales pressures). You could pay one of those companies for a support contract. But other than that, if someone else qualified exists, it's only worth their time to take these contracts if there's enough sustainable business to keep up-to-date with how Linux graphics drivers work; chances are there isn't, and they're spending their time in full-time employment doing something else entirely. So, you're at the same place where you started: the problem is complicated enough that there aren't enough reliable sales, and so it only gets done by large companies that can afford to fund the work because they already have enough sales of enough things.

(Full disclosure: I am one of the people who could have been qualified; one of the jobs I was considering a few years back was a graphics-driver-hacker position at Red Hat. I happened to instead choose full-time employment doing various, mostly userspace Linux stuff for a startup. When they ended their incredible journey, I would have loved to make a living taking contracts like the ones you suggest - and I still wish I could - but there aren't enough of them and they aren't reliable enough to make it better than just taking a full-time job with a profitable company.)