Actually... Just to be a little nit-piky
Plumbers USED to work really, really damn hard all the time. Back breaking work. Thats why they charge so much. The change though, is that because it was so hard - it caught to eye of innovators, and they build many the gadget + tool to rectify the problem.
Plumbers still work harder than the average desk jockey, but thats just the general nature of the job. The Job is ALOT easier now for them, but they sneeky [read: smart] bastards never put their prices down.
Can't blame 'em. They've got a sweet deal.
Though yes... There is a cap - simply due to work capacity [being paid by the hour/call] completable. But Half is cash-in-hand, and $150k is alot more than what most Business/Economics/Law/Desk guys are gonna earn [At least for first 5 -8 years]. Plus there is limited educational burdens [cash + time].
If the Plumber is a smart investor - S/he 'll be retired by 30.