I have heard of people buying up to 6 trousers for staple bespoke suits, like grey and navy, to prolong the use of their suit jackets. Lots of variation in appearance can be achieved by shirts and tie combination when wearing a suit. Those who get bored need not apply.
I've never understood that practice. But then, I've never worn out a pair of suit trousers ahead of the jacket.
6 trousers seems excessive. However, always wearing one's coat does not imply that both coat and trousers will wear evenly. The seat of the trousers may wear and become shiny over time, particularly if one often sits on leather chairs. It seems to me that the seat of the trousers would see the greatest of wear. More so than any area of the coat, unless one habitually leans on one's elbows.+1 I leave my jacket on during the day so in practice it wears out at about the same time as the pants. Elbows usually go first.
My grandfather always bespoke an extra pair of trousers, or at least an extra length of cloth, to keep for emergency repairs or the making of a new pair of trousers if necessary. My father, similarly, tries to get an extra pair of trousers when he can. I had always thought this wise.