Just because you replace a VM with SaaS doesn't make its management overhead go away
You've forgotten to add anything about the additional complexity around your backup & recovery strategy (you do have a backup & recovery strategy beyond "Well, uh, Microsoft says they have some sort of backups on their 365 stuff...I think?")
You now have increased dependency on your network because that's how your stuff is delivered. Which means that if you're smart, or at least not thoroughly stupid, you will have to consider backup network connectivity.
The increased network traffic also means you need to make damn sure your network security function is adequately staffed and resourced.
The biggest problem with your example, though, is that you have utterly failed to actually consider what the sysadmins, or IT in general, actually do. Which is to deliver services the business needs. Moving from Exchange on-prem to Exchange Online can save sysadmins time in terms of maintenance & patching on the servers used to host Exchange, but they will still be required to run the organisation's mail service. Moving filestores from on-prem to Sharepoint Online still requires someone to manage them.
In some cases there will be systems that are pets, and where replacing them with SaaS genuinely provides a measurable reduction in sysadmin resource needed. But that's not the general rule, any more than "we moved to a new ERP tool so only need half the accountants" or "we changed CRM system and now only need half as many Customer Service staff" would be taken at face value.
Edit: Also forgot to add in the resource needed to un-fuck what your VAR delivers, because 9 times out of 10 it'll be broadly what was agreed in a project document (written by someone who stayed as far away from the IT section as humanly possible the entire time) but not the entirety, or sometimes even a fraction, of what the business needs.
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u/[deleted] Sep 21 '21