I don't know why one should assume that songwriting as such would not improve with practice, the study of music theory, or exposure to more and more music. Most art forms and other endeavors do.

I didn't say that, at least that wasn't what I meant. I meant it's a mistake to assume that songwriting as such (either in general or for a particular person) does keep getting better over the years. And the same goes for a lot of other arts and other endeavours.

There are times when it does and other times when things stand still, and other times when they go into reverse. And it's pretty hard to be sure which sort of time you are living in.