I have a theory that dogs are getting more stupid.

Because well behaved dogs in good families tend to get neutered, because the family is responsible.
Meanwhile dogs in families that dont "care" and have misbehaved dogs tend to not care about them breeding.
This results in a large portion of mild mannered or smart dogs being removed from the genepool, and idiot dogs or even violent dogs having as many pups as possible.

Traits like low temperament or intelligence are removed slowly because the good dog thats in a good family gets castrated...

Also, since there is a lot of genetic bottlenecking, inherited mutations are causing dogs to become more and more autistic too.