Update 5/8/2024: We have released a more updated Roadmap Update covering the future on-premises releases of Exchange Server.
In September 2020, we announced that the next version of Exchange Server...
Updated Feb 12, 2025
Version 4.0
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I'll echo Christian_Kalz's comment about the end of support for 2019 and a short duration to then get onto Exchange vNext. Will the support date for Exchange 2019 be extended to give a proper upgrade time? I like cutting-edge technology, but the bleeding edge is not for my organization, we typically wait for the first CU (sorry everyone else, but thank you for field-bug testing!).
Following the guidance that you'll be doing those sorts of updates on a half-year basis, I'd be projecting the earliest I could consider upgrading to vNext to be Summer 2026. I'd probably want to move to new hardware as my current Exchange 2019 servers would be 6 years old by that point (off warranty, components more prone to start failing, SSD's for MCDB getting 'tired', etc).
I wouldn't want to have to take a hasty upgrade just to remain in a supported state from the software end, and I'm sure other on-prem customers are in that boat... even if in-place upgrades would be supported.