A rant about Microsoft and their On-prem stuff...
As one of those admins, I can tell you this: We were ignored; we fought it tooth and nail. but ultimately lost for a few reasons, at least for MS Exchange:
1) Security. getting "ZOMG PATCH THIS YESTERDAY" level security exploits on an application that's allegedly supported every month (and sometimes more often!) results in patch fatigue by the admin that's both overworked and underappreciated (and usually underpaid!) having to drop everything yet again to check and deploy a patch to a bunch of servers and deal with the resulting flood of complaints from people expecting 12 nines of uptime. It. Gets. Old.
2) Feature Rot. Seeing features that are uncommon, but used get unilaterally removed or broken by a cumulative update or security patch bundle with no notice or clear upgrade path, and having the next version of the application not have that functionality whatsoever also gets old.
3) and finally, a move to a subscription license for the on-prem application. If we have to pay the same amount for the on-premise email server that we are paying for the Online version, why bother with the hassle of the on-prem version?
We migrated our users once we figured out the 'migration path' for Unified Messaging (removed entirely in Exchange 2019, with Teams taking over that fully) and the ~200 shared voicemail extensions we have left in the environment are getting shoved back to Unity which we used to have before Exchange rolled out Unified Messaging back with the 2007 version. (It helps that Cisco relaxed their pricing for that platform finally.) It also removes the stress of wildcat exploits, emergency patches, and other mitigation strategies and general system administration of the underlying OS.
It also gives me (said overworked admin) time back for the other infrastructure projects I've had to put on the back burner due to the 'drop everything and patch now!!!11oneroneone' monthly hair-on-fire sessions, AND also allows me to tell our support group "user is licensed correctly for email, troubleshoot the client end and provide proof before bugging me with it next time" when people are using an out of date client.