I don't think the 76ers got worse. They got more athletic and better defensively. Josh Richardson is a huge upgrade on Redick defensively and Horford is one of the better defensive bigs in the league. If anything they stayed about the same, Richardson is very underrated.
I believe the reason he mentioned Durant's age is due the torn achilies. He is out for the upcoming season and suffering that type of injury in your 30's can be brutal. Nets aren't a championship contender in year 1 without Durant, and year two, they'll need him to be healthy and near Kevin Durant of old to legitimately compete for a title.
Horford is 33, but doesn't have that recovery from a serious injury, and his game has never relied on athleticism, so he's a guy whose game shouldn't suffer much as he gets older.