This is more of a perception driven narrative than a data driven one, but my impression is that Pogacar already showed a clearly stronger climbing level in the 2023 Tour than in 2022. That was partly masked by his preparation, and by the fact that Vingegaard was also exceptionally strong in that race. Pogacar seemed to carry that higher level into early 2024, improve slightly perhaps, and then by the 2024 Tour several riders appeared to be climbing at record strong levels.
I also suspect that Roglic’s apparent lack of “fatigue resistance” may be explained, at least in part, by his positioning difficulties in the bunch. On longer, harder, and more complex stages, that can plausibly contribute to his relatively weaker performances. It may not be primarily a physical limitation. In that area, both Pogacar and Vingegaard are excellent. So while some, like Politt, say Vingegaard is nervous, I do not think that is the right reading. Just about always makes the right split and wheel.
Addition: I also see many riders make a noticeable improvement when they reach 25-26 y.o. This is true lower down the results sheet also. I feel it is perhaps an incorrect assumption that we should expect improvement to be linear and marginal. For me, people appear at a new level, stagnate or hold consistent for a few years, then make another noticable step. Most often, the "big leap" comes around mid twenties. Last few years it's been riders born in 97-98-99. Now I think it will be 99-00-01.