Are there any good examples of this strategy? I can't think of anyone successfully moving a leading open source product back to closed source...I think normally it's the fork of the open source version that lives on...
The open source nature of it seemed to disappear overnight after Oracle bought Sun, but looks like Oracle's not doing too badly with it. I know there's the Open Source illumos (IIRC) but haven't heard of anyone using it in production.
We have been broadly happy with it and it has been quite stable after some initial teething problems.
Joyent? OmniTI?
https://www.joyent.com/smartos https://omnios.omniti.com/
SmartOS and OmniOS are both used in modern day production workloads.
In case of Microsoft, you don't even have to be very imaginative: how much of the stack currently in use by .Net developers on Windows is completely open? How important and hard to replace are the parts that aren't?
And how much of a difference would it make in the end to have had Core open source in the past, when the new and closed version is the only one that works with those billions of Windows10 tablets and those popular Azure services that everyone needs for their startup?