I understand and accept the reasoning of those people who claim that D&D and AD&D in all their variations were essentially the same game system, and that Third Edition D&D is not an iteration of that same game system.
That's a reasonable position.
I think they are wrong, but it's a reasonable position.
To me, even when you forget about questions of brand identity and fantasy flavour, Third Edition D&D is another iteration of the same game system as all the previous versions.
Races, classes, levels, experience points, rolling to hit, rolling damage, Vancian spellcasting, arcane and divine forms of magic, spell levels, alignments, saving throws, hit points . . . all of these, taken as a whole, are mechanical features which distinguish D&D from other game systems.
(Other games feature many of these features, but I'm not personally aware of any game possessing all of them that couldn't reasonably be called a direct knockoff of D&D, anyway.)
The original Dungeons & Dragons game from 1974 has these features. The Moldvay Basic Game has these features. The Mentzer Basic Game has these features. First Edition Advanced Dungeons & Dragons has these features. The Dungeons & Dragons Rules Cyclopedia has these features. Second Edition Advanced Dungeons & Dragons has these features. Third Edition Dungeons & Dragons has these features, in both its revisions.
True, Third Edition is definitely a more radical reorganisation of the game system than any previous version. It still, however, possesses all of the features that define Dungeons & Dragons, and they are recognisably derived from those features in all previous editions of the game.