You bring up a good point, actually: since Chrome OS is almost entirely open source as Chromium OS, couldn't just about anybody step in and provide a reasonable level of support for a fee, in the unlikely event the Google decides to quit?
The problem is that, for me, a huge part of the value of ChromeOS is provided by Google's suite of apps. Those apps are the reason it is viable in the business space. I would be looking for a way to run hosted versions of those in house and then we would have a really interesting situation.
I went to a Google Enterprise sales event and asked them about an internally-hosted version, but they were very clear that this wasn't something they were considering.
You could protect yourself against this by building an internal IMAP server and syncronising it with Google regularly. But that's a lot of work, and maintaining your own email server negates many of the benefits of outsourcing to a cloud service.
Could Google stop offeringing Google Apps at some point? It could happen. Tomorrow? No way. They have shown that they give people time and options when they no longer want to provide a service. Google Apps would be no different and they would probably even work a lot harder at making that easy.