Good point, the My List functionality is really annoying without the auto-removal. It's very cumbersome to remove items from the list on mobile so I end up with a list of mostly watched items with a sprinkling of things I may want to actually watch now. Not very useful.
There's some logic behind non-automatic removal: you get reminded about new seasons for things on your list. If you remove something and there's a new season, you will not be notified about it.