My statement isn't that post-3.0 D&D isn't a fun time. It's that maybe enough has changed that it's a different game from TSR-era D&D. Like as different as Savage Worlds is from Genesys.
Is that even controversial? The TSR editions are all fairly compatible with each other on a base mechanical level. Even if D&D and AD&D had to be "different games" for legal reasons, they're still not really "different" games.
3e/3.5/d20/PF1 used a completely different mechanical engine. Yes, it kept the same basic structure as AD&D 2e; borrowed some verbatim text (e.g. spell descriptions) here and there; and was, fundamentally, a reorganization and rationalization of 2e at its most complex (with all the options, like kits, proficiencies, skills & powers, combat & tactics, etc., "switched on"). But at this point, the argument could definitely be made that D&D3 was an entirely different game from what had come before. On a fundamental mechanical level, it just didn't play the same way.
And as for D&D4 and D&D5, these too are completely unrelated to earlier editions mechanically. "Full reboots" I'd call them, without even that bare thread of connection that D&D3 had back to late option-heavy 2e. You can't move characters between TSR, D&D3, D&D4, and D&D5 campaigns without
full mechanical conversion — and that's the most surefire sign there is that you're dealing with different games!
So as to the Thread Necromancy Question: did D&D "die" with TSR? Well, it depends on what you mean by "D&D." Is D&D a name-brand, a game-system, or an idea? Obviously the name-brand didn't die, but who here is such a corporatist zombie that they care about a name-brand before anything else? As to the game system — the TSR D&D engine — well, that's being kept alive by a dwindling pool of grognards and a decidedly more vibrant OSR scene that may not be growing like it was five or ten years ago, but which is at least firmly entrenched as a permanent corner of the RPG hobby. So the old rules aren't going anywhere.
But what about the idea? The
big idea of D&D is, in a sense, the RPG hobby itself, which is flourishing like never before. But the devil is in the details. The kind of game that Gary and Dave created before they even knew what they had created — the "fantasy wargame" that didn't even have the (at the time) pop-psych term "roleplaying" applied to it yet — is a
little idea that got partially revived by the OSR. But the OSR tends toward nostalgia and revisionism, and its dominant theory will always trump authenticity or rigorous reconstruction. People playing old-school D&D in an early 80s, B/X or AD&D1 influenced style — using old-school rules in an old-school way, but still just playing trad campaigns in small, fixed groups — are the bulk of the OSR. But what about playing in a 70s LBB style — the Lake Geneva or Twin Cities "fantasy gaming club," where lots of players run lots of characters in a persistent sword & sorcery milieu? That's pretty much dead. It might have even died back in the late 70s before gaining any real traction with the player-base that actually developed once D&D was released into the wild.