Being in the plumbing business for 47 years I can tell you that it has been lucrative.

We started out as a jobbing shop but moved into commercial, industrial and municipal work.

Our union A Plumbers make $75 an hour plus another $75 per hour paid into the union for benefits. This is for a 35 hr week.

So I try to encourage young guys that aren't sure of a career to look into the plumbing or steamfitter union.

The saying goes, "the crap in your bowl puts the food on my table" 😀

FWIW, if you're needing plumbing fixtures and you don't need some designer high-end stuff, I'd recommend Delta. Lifetime warranty. Working on a house remodel for my sister at the moment, it was custom built for her 35 years ago and had all Delta faucets and what not. Have no original receipts but I've replaced a bunch of stuff for free. She's out zero dollars for all the plumbing repairs I've done.

When something smells really bad, plumbers say "that's the smell of money"!
I don't mind doing some plumbing but bigger projects I will job out.

At work, we hired a plumber to work on some of our remodel projects and he would bring an apprentice with to work the drain end, so I would always refer to him as a supply side plumber!

Actually I like plumging. As a young guy I worked as the plumber for a girls' camp for two summers during Jr. college. I also did general construction there. They hired a very good plumber, but he was an alcoholic. The camp owner told me to work with him and learn as much as I could because he'd go on a binge at some point, & they'd have to fire him. So I stayed with him on plumbing jobs, did every dirty job he gave me, and learned a lot. One day he went into his room and didn't come out for three days. From then on, I was the camp plumber. My witching skills came in handy, since they had a lot of underground cast pipe to be replaced and didn't have a map for the septic tanks & fields fields.

Sounds good Marv.

One school holidays I needed a job and there was one for a trainee plumber. At the end of the summer I said thanks but no thanks and went back to school. But I learnt enough to do plumbing for the rest of my life. And they were pleased to have a schoolboy who could do brazing (in NZ we don't solder copper we braze it) and skinny enough to crawl under houses and fix things.

@NY65912, $150 net cost and then you pay workers comp? What in the world do you bill them out for? Gotta be $300 to 350/hr. Or more?

Funny looking back., before the basement floor was poured, the guy who was going to do the plumbing and septic field was tied up with a job and gave me basic instructions over the phone on how to get the septic lines layed out and sloped, passing under the footing. I remember him saying the pipe should lie "a half bubble off center". That was 44 years ago. No problems with that.
I watched his helper solder a couple of joints and realized that was no issue, so after the rough plumbing was completed I took over. Kitchen, upstairs bathroom, outside tap and water to the barn, which included a drain back to ensure the pipes were empty of water above the frost line.
It ain't rocket surgery.
The hardest job was extending the septic field about ten years ago. Getting the slope correct with my little tractor and loader, then getting the coarse rock in place before cutting into the existing field took some measuring, but everything came together as planned. Beginner's luck.

Getting the water to flow down hill isn't the trick in my view. The trick to plumbing, at least for me is to get consistent pressure and flow on the supply side by reducing pipe sizes and the opposite for proper ventilation on the sanitary side.

When I did my house it wasn't comfortable learning that math so I hired out the roughing and did the trims myself.

Absolutely. When I had to rough in the drains, everything was on site. I just had to dig some shallow trenches, put the proper pieces in their places, build a couple of enclosures around the inlets and cover everything else over again before the concrete guy got there the next morning.
Every time I use the showers, I mentally thank my plumber for his work on the rough plumbing on the feeds. Regardless of whether someone runs water in the kitchen, flushes a toilet or the washing machine starts, there is no change in temperature or pressure at the shower head.
He earned his money on that.

1/8" per foot

We used to go with 1/4” per foot. Is that too fast?

I was talking to a plumber several years ago, and he was telling me why they use 3 inch pipe versus 4 inch pipe. Low flow toilets don’t provide enough water for the 4 inch pipe to push the solids.

My repeating issue with plumbing here is the MIL washing her dentures of the polygrip into her bathroom sink. Every 6 months to a year she complains about it draining slow. I can usually get it unclogged - but twice in the last 4 years I had to call in our plumber. He uses a power snake that reaches further into the wall behind the cabinet which must be where it consolidates and blocks the water flow.