Cheap workwear or army-surplus trousers work well. If you are a fashion plate such choices may not suit.
I usually get about 3 years out of a pair of German moleskin trousers (which are vaguely jeans-like) before the crotch tears and needs patching. I wear these all the time, not just for cycling. But I ride quite a bit.
After about 5 years the backside goes as well - that's terminal. I then cut them up for patching other pairs.
Dickies polyester work trousers are effectively indestructible (but rather warm). Only pair I ever had to bin was due to fire damage[1]. About £13 from Screwfix and similar places.
[1] Aeons ago. Stick welder involved. Oops. Never was much good at welding...