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."