First, I have never had a plumber come to my house that I wasn't charged by the hour. Things may be different where you live but I have had this experience in 3 states with numerous plumbing companies in each state.
I have had some cut me some slack by not charging an extra hour for 5 minutes or so but anything over 15 minutes I have been charged an hour.
Second, the question wasn't how much can a plumber make working for a plumbing company.
A plumber in my area, working for himself, charging an average amount, before expenses and taxes, working an average year, no overtime, no weekends or holidays would make about $156,000.
I answered the question, I didn't assume anything else, whether he worked for a plumbing company, whether he had others working under him, etc.
And if a plumber (a plumbing contractor) bids jobs for a construction company, he may earn much more than what he would make on an hourly basis. Or he could lose his shirt.
Some company’s only round up to the half hour or rarely the quarter hour, not always the full hour as you said.
I don’t think that you know much about the service plumbing business, but you do keep throwing more and more made up figures out there.
Next time you eat breakfast out, estimate the diner’s gross, ignore the owner, and then go tell the short order cook how rich he must be based on your figures.