I'll give you Nibali. Purito is a very peculiar case, where he was a solid hilly classics rider without any good GT performances until he was almost 30. Then he sort of stepped out of Valverde's shadow, especially after he left Abarca. But he also never won a GT.Nibali gradually improved until he was 29, not too dissimilar to Basso. Purito is an even more unusual case, but his rise was quite gradual.
The gradual improvement until 30+ was mostly a thing in the days of extremely thick blood, where Basso belongs. Pre 90s and post blood passport it hasn't really been as prevalent. We're more back to big talents revealing themselves early, and part of the reason might belong in the other part of the forum. But as you point out, of course there are exceptions.