And I personally believe that that is precisely what it is. It redesigned most classes, changed when you get class features, completely rewrote feats, rewrote or modified several spells, you don't get ability scores from race, etc.
The rewriting of species and background has been a pretty annoying issue, actually, because every VTT I've used (only three, admittedly, but still) will simply screw you over if you try to run a 5.0 background but a 5.5e species. You literally just...never get the chance to have the extra points.
Given that that means you actually do need conversion stuff and cannot simply mix rules components, I don't see how that can be anything smaller than what 3rd edition did, what with re-writing multiple classes, changing spell levels and access, and rewriting the wording of various spells to close known loopholes.
If "2024" 5e isn't different enough to be 5.5e, then the edition that gave us the term ".5e" wasn't big enough to be 3.5e.