Transforming has been a Druid thing since day 1.
Plenty of things come and go with time. Druids in 2e, a least, didn't get the ability to change shape until 7th level. Now it's innate.
Plus, elements of transformation have been present in the Sorcerer since there
was such a thing. Remember the Dragon Disciple PrC? Yeah. That's literally what they were rolling with--but with the notion that that would generalize to many different kinds of sorcerer.
It would actually make your sorcerous soul
matter, rather than it being 110% purified fluff with zero relevance beyond a couple middling class features, which is what we actually got in 5.0. 5.5e has tried, vainly, to squeeze something more interesting out of the withered husk of a Sorcerer we got. It's...pretty thin, and doesn't really
do much until you get to very high level (well, other than the bonus spells, but those are more a convenience than a class feature IMO.)
Playtest Sorcerer was interesting but wasn't a Sorcerer.
You declare this like there was a clean, consistent, universal pattern across many decades.
That's simply not the case. The sorcerer has been something fairly different in every single edition where it's appeared--which, I'll note, is only 3e, 4e, and 5e. Indeed, the 3e and 5e Sorcerers are actually some of the
most different classes between those two otherwise highly similar editions.
It was perfectly workable as a new Sorcerer for the same reason that 3e
inventing a Sorcerer in the first place was perfectly acceptable. There simply
isn't a single fixed form for it.
Just like how Druids picked up animal companions in 3e, and then lost them again in 5e. Or how Bards have been literally like
four different things across the various editions at this point, with almost nothing in common between them beyond "using the magic of music". 1e Bard was functionally a PrC, a hybrid fighter/thief/druid with extra stuff added o'ertop. 2e Bard was functionally a prebuilt Wizard-Thief with some musical abilities. 3e Bard was a hybrid Rogue-Cleric-Wizard that sucked at being any of those three things (except maybe Rogue) unless cheesed, and was generally a weak class unless you stuck to E6 type rules. 4e made Bards awesome, being trickster/healer/drum major/renaissance (wo)men. One of the few things 5e genuinely translated from 4e was the Bard, mostly because it was the first time since 1e that they'd actually been clearly
good, hence why I say it's only been four different things rather than five.
Classes change between editions. Sometimes, the changes are small or subtle or not clearly understood until later. Sometimes, the changes are dramatic and obvious and all that you have in common between them is the overall loose thematic notion.
The D&D Next playtest Sorcerer was more similar to past Sorcerers than any post-1e version of Bard has been to the 1e Bard--to the point that 3e literally DID try to revive the old 1e Bard as a legit PrC, the Fochlucan Lyrist (but, unfortunately, like the 3e Bard, it sucks and isn't as good as the
actually good Bard PrCs, like Sublime Chord). Yes, I agree that there are differences. The differences are not nearly as dramatic as you claim they are.