So I can't disagree? I have to be wrong because you experienced otherwise?
If you are claiming that it's unplayable as a general thing, yes you have to be wrong because I experienced otherwise. If you're just speaking for you, then I accept that it was unplayable for you.
I could point out that the actual 3e playtest didn't really cover those levels and Living Greyhawk feedback really didn't approve that range with 3.5e either.
Not relevant. It was playable for me and literally several dozen others that I saw in my immediate games and know of several dozen more, so what happened before that doesn't matter. I've only met a few who think like you do.
I could also point out the absurd variance in numeric capacities at higher levels, the lack of material support, the effort it takes to make the game function because of this lack of support and wide variance in numbers are problems that reduce playability.
If you are talking epic levels, then sure. Lack of support(at least good support) was true. If you're talking 12-20th level, there was literally TONS of support.
I could also point out the necessity of magic and magical countermeasures at high level reduces the range of scenarios and protagonists quite a bit outside of fiat which makes the game outright unplayable for some character types or making them rather secondary to the play experience.
I thought you varied things up when you DM'd it. If you had, then that would not be an issue.
I could also point out that there are literally polls on this very forum that chart where people (both DMs and players) ended their campaigns in 3.5e but I don't know... even when they trended to a level 11-15 cutoff, would that convince you since the number of voters are so small as to be irrelevant?
And now we are back to correlation does not equal causation. Most games ended early because people moved, PCs died, life got busy, etc.
I could point out that the community has recurrent conversations about the lack of high level D&D games (as opposed to the opposite) and while everyone seems to remember real life intrusions, fatigue, and schedule conflicts as the reasons those games do not occur, the reason people conveniently forget is that the game is unwieldly and unbalanced at those levels.
And I can point out that the vocal minority is usually the voice most commonly heard, because the happy folks don't bother. I could also point out how the vast majority of players don't come to forums like this.
You don't have an acceptable or unbiased sample. So sure, there were people who felt the game was unwieldy and who don't like game that are imbalanced. All of 3e was imbalanced by the way. Not just the higher levels. The existence of those people, though, still can't possibly make what you are claiming true. The existence of the great many of us for whom the game worked just fine up to level 20 disproves your claim as a general thing.