Something interesting I learned when looking into all of this is that if you have a large environment (2000+ mailboxes) and transition to Exchange Online, Microsoft still (since 2010) has no idea on how to fully decommission your Exchange Server environment, since you need at least 1 to facilitate on-prem AD connectivity (which isn’t true if you didn’t have a hybrid environment). So even if you transitioned to the cloud, you may not have been safe.

https://docs.microsoft.com/en-us/exchange/decommission-on-pr...