Comparison, and being able to open an image in the version I edited it in if there’s a problem in the new one, is the only reason I want to be able to run an old version, and not allowing this after an upgrade (in a more formal way than it happens to work now) is too restrictive IMO.
Capture One and Exposure (for two) don’t seem to have a problem with this. They don’t even mention upgrade in their license that I can see: it’s just a new license for the new version, not a replacement for the old one. (In the case of Exposure, they list both X4 and X5 as licensed products after my recent upgrade.) They also have more robust activation management though, that allows you to activate and deactivate from within the software, instead of having to contact DxO support to reset an activation counter manually. The software itself also deals with running multiple versions since sidecar files are written to version-specific directories, so that different versions can’t step on one another as with Photolab.