The people who lease toilets to people are landlords, as part of leasing the rest of the building or unit.

I'm pretty sure plumbers do sell service plans to businesses. It's just that households don't really need regular work.

Service for what? I'm in my 40s and now that I think about it, I've never had to call a plumber in my entire life. The only time I can think of is my parents calling one when they got a new dishwasher 25 years ago.

I've had the plumber out twice this year. Probably depends on how handy you are with plumbing issues. Me not so much.