I think you make a good point regarding support. This is, for businesses, the crucial issue. They want to buy reliance, support and certainty. This is what commerce, like Microsoft, offers; peace of mind for (big) bucks.

Organizations can't easily take measures to assure proper support for a lot of open source software. They'd have to hire and probably educate a lot of expertise, which all has to be managed too.

It's just a whole lot easier for decision makers to spent extra money to have a contractor solve any issues, or at be able to blame (sue) them.