If the answer is yes, OSS has mostly been an alternative option but not the first choice. (With all due respect and deference to the work put in by OSS GUI teams!)
If the answer is no, and especially if it's a shared need but not the primary product (e.g. kernel, web server, intermediate processing step), then OSS has taken over the world.
I think Cisco still uses proprietary software too.
I'd say that the cutting point is when no new project considers Oracle as a viable tool. This time has largely come already.
MySQL and Postgres are kind of nice, but not really the same league at tooling level.
If the database is primarily a data store, unseen by the end user, then Postgress, Firebird etc are fine. If however the database is an active part of the development stack, being used by lots of "users", writing new code etc, then commercial offerings with better tooling win.
> The pleasantness problem deals with the question how satisfactory a system meeting a given functional specification would be; the correctness problem deals with the question whether a given system meets that functional specification. The correctness problem being an entirely technical one, the functional specification can provide strong heuristic guidance for the system designer.
Actually needing to sell your product eventually/get people to use it seems to be a fairly necessary part of ending up with a good UX.