I have personally seen checks cut to CoreOS that were larger than the D&B number you quote. The set of similar corporate customers we'd run into was surprisingly vast and diverse. Redhat's made a brilliant move here.
For $250M Red Hat bought outstanding technology, a strong R&D team with community street cred, and upstream influence in Kubernetes. They know how to monetize these things. They definitely did not buy revenue or customers. In fact they got a great price precisely because CoreOS failed to monetize these things sustainably, and needed a way out. It's a great outcome for the team, and a great match for Red Hat.
Redhat offers an existing and proven enterprise customer base that will cut the big checks. Redhat customers are the customers that CoreOS always wanted but likely struggled to acquire.
Assuming the industry norm, which is a botched sunsetting and incomplete migration, the majority of free CoreOS users would go looking elsewhere. And they would probably scatter to the wind rather than flock to a single winner, since the OS space is quite crowded and fragmented.
My point was not to imply this is a bad deal for any of the involved parties, but simply to add a data point to the discussion, one which other entrepreneurs looking for areas to add value should find highly interesting.