Don't get fixated on plumbing itself. The point is if a bunch of people rush into any profession it leads to wage depression. Unless the amount of plumbing needed increases, the overall amount of money flowing to the plumbing populace is likely to stay roughly the same.
> The point is if a bunch of people rush into any profession it leads to wage depression
Eventually. Wage depression does not happen linearly. You're asserting that demand is maxed out and there's no more money to go around, and that's just not true. A lot of people just don't bother because tradespeople are famously difficult to work with because they are so overbooked.
It takes a week because if you want it fast they charge you an emergency rate. This aspect of the tradesman is independent of demand and one of the perks of their lines of work much like over time in other fields.