I consider anyone who thinks otherwise an amateur and don't trust their judgement.
I would give some leniance in very special situations where there is a clear, thoroughly planed migration path to FOSS and the problem has very good reasons to require a currently existing tried and true proprietary solution that is cohesive and losely coupled to everything else in the company. Something like that however requires deciders to know what they're doing and those joints usually have their systems under control and can tell a vendor to go and f*ck themselves with confidence if anyone gets pissy or turns up with a fresh absurd future pricing table. Organisations like these usually are 95% FOSS already and basically follow the same rule stated above, for obvious reasons.
Those who don't get 20+ years of this VMWare, Oracle, SAP, whatever sh*t we've been seeing ever since the dawn of time.
Bottom line: Mission critical? FOSS only. Break that rule and I'm getting ready to cash out of your gig ASAP.