> This is just a general argument against constant prices for everything though.

Maybe EVERYTHING shouldn't BE "constant prices". Maybe where there are practical alternatives to constant pricing, those should be preferred and used.

> Charging $10 for a t-shirt is regressive.

No. Not unless there is only 1 type of t-shirt in the world available. If I'm poor I can go find cheaper t-shirts either less stylish, poorer quality, from a generic brand, from a discount retailer, second-hand (used), packaged in bulk, etc., or maybe wait around for a sale on the t-shirt.