> One assumes there is a relatively fixed supply of plumbing problems.

The volume of plumbing problems is going to increase at least linearly with the housing supply, which (theoretically) increases more-or-less linearly with population. Plus, people move around, so there will be concentrations of problems, and those concentrations will move with the population.

That doesn't account for how technology and building codes and material changes have made previously simple plumbing problems more complex over time.

On the other hand we now have waterless urinals. Maybe plumbing tech and equipment costs have eliminated some jobs and made some jobs less skilled as it cheaper just to replace than do repairs that take knowledge.

Software jobs are many X what they were 30 years ago, while I doubt plumbing jobs have change that much above what they were relative to the population 30 years ago.

House sizes have doubled over the last 50 years. I would not assume a fixed supply of plumbing problems.