I wear a lot of cargo joggers, which are really the best of both worlds. 10/10. I usually have to buy them in men's because of the eternal struggle for pockets, but it's worth it.
Not sure that they replace underwear but my wife has these and still wears very thin underwear with them since the stretch mesh you are calling underwear is not really underwear nor super comfy with direct sensitive skin contact during exercise.
I splurged on some "nice" sweat pants, and that's pretty much all I wear for fall/winter/spring, when I have to leave the house. Cargo shorts for summer.
It used to be PJs all day every day, but those nice sweats were a game changer.
Isn't sweatpants what most of the "younger generation" wear? Baggy T-shirt, sweat pants and sneakers with socks pulled all the way up outside the sweatpants.