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.