I'd take Kotikoff's research with a grain of salt. His analysis was done during the midst of the housing boom, which latest from the late 90s through the late 00's, where new homes and new condos, and gut-rehabs demanded A LOT of plumbing.

Our housing starts are less crazy today. I imagine lower demand goes hand-in-hand with lower wages.

This is just conjecture on my part though. I'd be happy to be wrong.

Circa 2011, in a low population area, I had to call 4 or 5 plumbers to find one that thought dealing with a clogged toilet was worthwhile. One was busy, another thought it was too far a drive (~30 miles!), another said it didn't sound like a job he wanted and the fourth didn't get back to me before I had found someone else. Who knows if I got burned, but it was $350 when I did get it done, so maybe worth that extra 5 miles of driving.

(roots had (probably) grown into the line to the septic tank)

On the other hand, lower reimbursements and higher tuition have also made medicine less attractive since that time period.