It’s already happening, google “Cloud Repatriation”
That is a gross simplification of the issue with Exchange.
What is happening with Exchange is the new version of it will be subscription. That is, if you want to run it on-prem you pay a subscription fee.
However, it’s dangerous in the extreme to run an older version of Exchange server that is no longer supported by Microsoft with security patches, because Exchange is one of those very few corporate servers that MUST have unfettered access from the Internet for remote clients to access it. So with “traditional perpetual licensing” while it was possible to setup something like an Exchange 2008 R2 server and run it literally forever (a modern day Outlook 2024 can still connect to such a server) the only safe way to do it is to require every single PC, Mac, iphone, Android phone, tablet, etc. that your users may be using to run a VPN into your network because you must completely close off access to such a mailserver from the Internet otherwise it will be broken into in minutes and used as a platform by criminals to send phishing and scams to the rest of the Internet. And that’s completely impractical.
So the upshot is that while the “old” licensing was “perpetual” that was in name only - in actuality you STILL have to periodically buy new versions of Exchange - as well as new versions of “client access licensing” for your new version of Exchange.
Microsoft has not as of yet announced pricing on the subscription version but it’s suspected that it will be set to be equivalent to pricing for Office 365 accounts that are run in the cloud. And it’s also guessed that there will be a point at which the number of users make it less expensive to run Exchange SE on prem than go to 365 accounts.
Note that it is common knowledge that all email in and out of mailboxes on Office 365 servers in the cloud is scanned by Microsoft for marketing data which they then sell out the back end. Needless to say it’s also common knowledge that Law Enforcement and various government agencies do not need to get subpoenas when they want access to user 365 mailboxes, they just ask Microsoft and are given it. If you as an org are perfectly happy with having your data available for scanning by the Elon Musks of the world, then give up your on premise Exchange server and bend over and let everyone and their dog up in your business (well, bend over and let Microsoft let everyone and their dog up in your business) without your knowledge.
The large Fortune 500 orgs know this, even if the small fry Ma and Pa Kettle companies don’t believe it, and have told Microsoft in no uncertain terms if you get rid of on-premise Exchange we will switch our stuff over to Linux mailservers. Outlook already supports IMAP and there’s a 3rd party commercial product that is not much money that interfaces CalDAV and CardDAV servers (like Radicale) directly to Outlook’s contacts and calendaring. Which is why Microsoft will NEVER get rid of on-premise Exchange, nor will they ever be able to raise prices on it to the stratosphere, since the free Linux alternative (postfix/dovecot/radicale) can be used to emulate an Exchange server for Outlook clients. So don’t worry about that happening.
The issue with Cloud is that it only makes any cost sense when you have a cloud provider that collects up a large number of “Ma and Pa Kettle” small fry companies - we are talking 50 seats and below - and puts them on a shared infrastructure. The reason is that the overhead for running a solid server is basically fixed, and that fixed cost has to be spread over your users.
So a $10k server platform and infrastructure is $1000 per user for a 10 person company. However, it’s only $33 per user for a 300 person company. Now you see why the small fry are going to the cloud - even at the current extortion pricing of Office 365 - it’s cheaper for a small 10 person company to do it. And that’s not even discussing the cost of skilled labor to setup that $10k server platform, which a 10 person company likely cannot afford, anyway.
But the large orgs have so many more users to spread the infrastructure and skilled labor cost over that it makes less financial sense the larger they get. Maybe if they are, for example, a hotel, where they don’t need to provide IT infrastructure for their guests other than throw a cheap crappy wifi AP in the room and a phone - that might fly. But not if they are a full blown company that needs IT staff to support user PCs and applications.