Why is it stupid?
It quickly and cleanly communicates that 3e and 3.5e are fundamentally the same game, with iterative updates. Just like how the community has taken to calling "blending 3e, revised or not, and Pathfinder" by the term "3.PF," recognizing that PF1e was ultimately just very extensive tweaking, not a wholly new game. (And part of what made me willing to actually hear out the devs on PF2e was that they admitted that mere tweaking cannot fix the fundamental flaws of thr underlying 3rd edition ruleset.)
Part of the reason this works so well is that it works exactly like something many end users are very familiar with: software version numbers. Windows 10 is different from Windows 11. But Windows 3.1 is the same operating system environment as Windows 3.0 was, it just has some more features under the hood. FFXIV patch 6.4, which just recently came out, is fundamentally the same game as 6.0, but differs from 5.0-5.5 (the Shadowbringers expansion) because the classes ("jobs") no longer work the same way. If you played during ShB, but waited until now to pick up Endwalker (6.0-6.4 thus far, 6.5 coming late this year), you'll need to relearn some things. Much will still be familiar, but much has also changed.
So...what is so horrible and (as someone else said) "deceptive" about the X.5e label? It follows models from other, well-known things, is quick and clear, and emphasizes that the changes, while important, do not actually make a break with the fundamentals of the game being "revised.