You are absolutely correct! Mail should never be anywhere near "hundreds of GB". I made a mistake in posting that. But if each of 3 users has 3-5 GB over 4 years that will add up.
I also agree that there are millions of MACs that run fine for years without maintainence. Depends on how it is used. I am not sure how rare it is to not repair permissions if you have family users accounts, especially kids on your Mac.
Perhaps I exaggerated the sluggish. New out of box, my MAC started up in seconds. Over time, new apps added, start up took longer- perhaps close to 2 min. I had to remove some apps from start up menu to regain the faster startup. For me that was maintenance. Over time adding and removing apps leaves traces in Libray and preference folders. Not a big deal usually but sometimes a software upgrade doesn't work because an old file is still hanging around. Plus with fast user switching enabled, all those open programs at start up affected things.
I admit, I have an organized uncluttered personality but I don't think I am alone in maintaining a MAC.