It is easy enough to dump on Razz as most of you have ably demonstrated. That is not the same as refuting his underlying point, however much you feel it can be refuted.
If I may paraphrase or reinterpret the OP question - after how many editions, wherein D&D sees significant rules changes sufficient to justify a news edition, does the term "D&D" become meaningless as definitional of any particular rules set?
Leaving aside OD&D (YMMV

), 1e set the bar as to the rules that defined the "D&D" experience.
2e changed the rules, certainly, but did it do so to such a degree that the D&D experience of 1e was no longer recognizable as such? IMO, it did not. 2e was different but still easily recognizable as derivative of the 1e experience.
3e again changed the rules. It changed the rules in a much more thorough going way than as between 1e and 2e. As between 1e or 2e, and 3e, 3e was virtually a different game that played entirely differently. It was still called D&D but it had fundamentally changed.
4e promises to again change the rules. At one of the D&D Q&A seminars at Gencon, the comparison was made that the change from 3x to 4e would be more akin to the change from 1e to 2e than from 2e to 3e. Still, this pushes the rules set further from the 1e baseline. And therein lies the rub.
1e is only the baseline from an objective standpoint (leaving out OD&D at that). Subjectively, 1e is not the baseline if you never played it. For people who came to D&D with 3e, 3e is the baseline!
So has D&D "jumped the shark," if you are capable of addressing that question without the snark (snark requires no thought, just attitude)? I think there are multiple answers that can be defended.
If you started with 3e, no, D&D has likely not jumped the shark with 4e from what we know at this point.
If you started with 1e, yes, D&D has jumped the shark with 4e, from what we know; actually it jumped the shark with 3e.
All this said, it is a different matter whether you care that D&D has now become something significantly different from what it once was. Obviously, many do not care so long as they get a game they enjoy. What is enjoyable or not is, of course, entirely subjective. What is objective, to the OP's point as I read it, is that D&D with 4e will be sufficiently different from 1e or 2e as to be virtually a different game, even while still called D&D. Whether 4e will be as different from 3x remains an open question but that it will be sufficiently different to justify a new edition is, IMO, beyond doubt.
The sky is not falling. It is changing its oxygen/nitrogen mix. Some are going to have difficulty breathing the resultant atmosphere. Some will breath in with gusto. To some it will smell like lillies, to others sewer gas. Acknowledging such is not the same as taking sides.
But of course, snark on!