Software is never perfect. Providing the ability to install multiple versions of ArcGIS Pro side by side would allow users to test new versions without having to completely uninstall and reinstall each new version.
Impressed that @SimonSchütte_ct 's workaround for this (frankly rather significant) problem works. It's frustrating that Python versions are apparently the issue here- one would think that this could be addressed by installing per-ArcGIS Pro major version Python directories.
A workaround I have found is to use the 'Sandboxie' program (Sandboxie Classic works, Sandboxie Plus may make it easier to manage). Without ArcGIS Pro installed (having had it installed previously shouldn't be an issue), you set up a new sandbox, install the version ArcGIS Pro you need, required patches, and any required plugins from external vendors into that (this can be done by running the .msi's and .msp's with 'msiexec' from the Sandboxie interface). After this is done you could either install the latest version in its own sandbox, or, just straight on into the normal Windows machine.
The primary advantage to doing things this way, over running ArcGIS Pro in a VM is that GPU access will be conducted directly as with running it 'raw', rather than a GPU being emulated (slow) or having to have expensive cloud costs or do GPU passthrough with the attached virtual desktop infrastructure, server GPUs and Nvidia GRID licensing etc. Disadvantages are that if one needs to install these versions onto multiple machines throughout an enterprise, it's going to be more difficult than just serving up a virtual desktop or virtual application that's installed in one place (though it should be possible with the appropriate use of the command line tools), and that Sandboxie is, effectively, security software, and like other security software may have incompatibilities with other security software installed on your system (though I've noticed no issues with Microsoft Defender).
This would be incredibly useful for our organization as well for all the reasons already listed here. We work with vendors who still need Pro projects delivered in v2.9. We also have run into the Enterprise/Pro versioning issues.
