Long, long ago when I worked at Nortel (a now defunct, but then huge telecommunications company), they used to pay millions of dollars a year to Cygnus to support a particular embedded version of GCC. This, despite the fact that Nortel had more than 10k programmers on staff including a compiler team!
I think the real reason these support contracts exist is because companies (even large ones) don't want to dilute their focus maintaining projects that are peripheral to their core business. It's not so much a technical problem, or a money problem -- it's a management problem. They can't scale out to handle every little thing.
I think OSS is a red herring in this conversation. Most companies just don't care about that. They don't want to support it themselves (even if they are big enough to do so), and they need to have confidence in the company that provides the support. Build that company (hint: you need to be sales heavy!) and you could sell Postgresql just as easily as any other database. Of course breaking into an entrenched area in Enterprise software is always going to be difficult, so I'm not sure how successful you would be with this particular product, but you get my point, I think.