I've called Microsoft maybe 3 times in my career. One was for an on-prem SharePoint farm issue (User profile sync issue, took 8 hours but Microsoft was able to resolve this one), one was for Windows VSS (onboarded a client with some really funky VSS failing/inconsistent/time out issues like a writer wasn't releasing during backups, which were then failing intermittently... Microsoft was unable to resolve this one even using their own backup software). Not sure what the third one was though.
The extent of support is software only. If I'm using StorageCraft to backup a server, and backups are failing, Microsoft should be able to load Windows Server Backup on their OS and get backups to succeed. If they can't get it to work and recommend a rebuild, that's a problem for me. A rebuild is something a provider would do because they don't understand every single component of the OS to be able to resolve issues; it might be quicker, sure, but that's not the point here and is a separate discussion.
The point is, there are times in people's careers where they onboard a client who have systems in a problematic state, or currently supported systems that just break. Microsoft (in this case) should be able to provide any and all support to be able to fix their software (whether it's the OS or applications) without requiring a reinstall. In the cases where Microsoft support was unable to find a resolution in a timely fashion, I had to revert to reinstalling applications and/or the OS.