I remember thinking this about Chris Paul before he was traded to Oklahoma City. I think he had one bad year in Houston (judging by his stats) and people must have freaked. As I recall he also was frequently injured.
However, the conclusion about Blake Griffin a couple years ago proved accurate. He has been absolutely sapped of his athleticism, and his ceiling is now "useful role player on a super team". So, some of these "washed up" assessments turn out to be correct.
The problem with Kemba Walker is that his game is predicated on quickness, explosion, and violently cutting and stopping. That's not at at all conducive to chronic knee issues. This is different from a Chris Paul, for instance, who's a bigger-bodied PG who still has enough athleticism combined with his brain, passing, and leadership.