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 sudden

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.