You're actually wrong again, they were 9-7 then 6-10 after he left .... keep going, then they were 2-14 drafted Mariota then went 3-13. He left them at 6-10, so one better year in the next 4 made them markedly better than when he was there? I don't think so.
I'm not dying on any hill, just merely pointing out you're absolutely wrong about him being one of the worst ever. The moment you say - I didn't really mean ever - then I'll stop.
With his only good QB (McNair) they were good but he's consistently fielded teams that were a b**ch to play against. It was no different in St. Louis - he got that defense playing nasty and that's what first started turning heads their way.