I have a theory regarding Kevin seeming to get dumber as the show goes on.

As the people you're around begin to get more comfortable with accepting who you are, they place you more into a box. Everyone just begins to accept him as stupid and helpless and act accordingly. When things are done for you and people expect less and less of you, you begin to rest on that expectation and also act accordingly.

There are exceptions. Depending on the type of person you are, you may resent it and do more to prove yourself, but Kevin being the type to appreciate having to work less, it makes total sense. Instead of being like "hey, guys, I'm really not as stupid as you make me out to be", he embraces it because it's easier and I think he gets a kick out of it.

It used to really bother me too, but since I adopted this perspective, it really makes me feel better about his intellectual decline. I know I could be adding substance where there is none, but it helps him remain one of favorite characters. Lol I hope this helps revive some of the Kevin love to someone else. 😊