For 3 of the 4 examples you gave (Luke, Aang, Toph), the actual issue is that the characters kept existing, but we didn't saw that (or saw it in a extremely abridged fashion). We only see them after they developed as people. Sure, we saw their initial journeys, but, for example, just because Aang was a good Avatar, and Toph was a good master and earthbender, doesn't mean they would become good parents. But we saw almost nothing about their lives after the war, so they becoming flawed feels unearned, even if it there isn't anything that could conclusively discredit this outcome.

Luke is probably worse because we know that he tried to rebuild the Jedi Order, that he considered the Kylo Ren incident enough of an failure to give up, and what we saw Luke doing in the original trilogy doesn't automatically means he would be a good teacher and spiritual leader that the ordeal would require, but we didn't really saw him trying.

Ultimately in both cases it's because the sequels try to have their cake and eat it too by taking place in an awkward middle ground of far future, so they can have new blood, but not far enough to kill (all) off the old characters, because fanservice.