But I have 90s t-shirts that are just now dying after all these years of being dried only in an electric dryer, and other t-shirts just a few years old that are disintegrating. There's definitely been a quality change in the average shirt.

Inflation halved the value of money since the 90s. If you haven't been paying double for your shirts then the quality hasn't changed but your price expectations subtly did.

We see this everywhere. Manufacturers moving to more disposable products to keep the average prices within consumer expectations. Shirts and Cars certainly ain't "what they used to be."

In New Zealand, culturally people generally use dryers only when it is too wet to hang them outside. Dryers are seen as wasteful and destructive. T-shirts last longer but they do not last forever. Quality has gone down substantially.

Yes completely agree. I always hang up my washing (also in NZ, don't have a dryer) and was recently sorting through my tshirts as we are moving country. I have one t-shirt that is nearly 20 years old and still holds its shape (though the color and print has faded) on the other hand i threw away a bunch of other t shirts which were just over a year old because they developed holes and particularly the collar is completely broken. Funnily their color and print is mostly fine.

I don't think brand is a good predictor either, e.g. the old t shirt is from threadless IIRC while I had many other threadless tshirts which didn't last near as long.

Cotton shirts aren't valuable enough to treat this gingerly. I hang dry my merino, but it's easier to just buy new cotton shirts every five years or so. That's a good run for clothing.