I call BS. I worked for a state government that bought a piece of license management software for a handful of millions of dollars. The company announced they were ending support for the project almost as soon as it went live, leaving our IS people with barely functional software and a mess of poorly documented code.
Another company I worked for purchased accounting software and support for it from a company that was gobbled up my Microsoft, then suddenly found that their support dried up, leaving my company's one IS guy with barely functional software. You may have heard of it, it was called Great Plains when they bought it.
Nothing prevents a company selling closed-source proprietary software from going belly-up tomorrow, or simply deciding that breach of contract is easier than software maintenance. On the other hand, all you need if the code is open is a few people interested in the project, or barring that you can always hire a coder to make fixes as their needed if you have something obscure enough that the foss movement isn't already doing it.