That may be true in theory, but how many people can actually fix a bug they found in Linux, or MySQL? Very few.
If the software is proprietary, good luck with that. The proprietary originator might have even went bankrupt and the source code lost in the chaos. Hell, the whole idea that you'd trust parts of your critical business systems to a random external vendor with different business interests is crazy.
And open source projects can be abandoned with the business user left with a million lines of source code and no idea what to do with it.
Actually, this exact reason is why many of the larger organizations use FOSS over proprietary software these days. FOSS gives a piece of mind that is impossible with proprietary software.
The reason that many large organisations use FOSS over proprietary software is that they don't have to pay for it. Idealism rarely has anything to do with it.