I guess it come down to the point of people that don't look after or keep fish will never know and the easiest way of describing overcrowding is 1 adult and 20 kids and there's only going to be a certain amount that can be cared and kept happy before its to much of a problem that they'll die And you can never really know if they'll be looked after properly as you wont be there to make sure of it
My mother kept fish... but definitely not the right way (and my fail skills of keeping fish during the last 17 years till 2012 speak of that, I've learned them from mom...)
She used to have one tiny fish tank of maybe 40L, not sure exact size but it's definitely small and damaged. She had a crowded tank with angelfish, guppies, swordtails, mollies and platies. She described it as being FULL of fish, so I can only imagine. And...
no filter. Water changes maybe once a week, maybe once every 2 weeks... She said she even left for a month or more and left her fish with
no food or water changes or a filter. And yet somehow still found surviving fish in there. Maybe she didn't even count them.
Several years later, when I've received my second fish tank (first time it was a very VERY tiny tank with 4 male guppies in it and no filter and my mom killed them after putting a decoration in soap and not washing it properly), I had a tank similar to what my mom described... only with more fish and it was maybe around 50L... And probably 2 feet long.
I've kept 4 bettas (in pairs), a bunch of Zebra danios, guppies, swordtails, a platy, two black lyretail mollies, four baby angelfish, some gold-colored tetras with a black dot on their backs (no idea what they were), black skirt tetras, neon tetras... and my dad had to add 2 hoplos to that. -_________________-
Oddly, I managed to keep those alive by changing water 100% once a week, with no filter. But during the summer, all the sensitive fish died, leaving me with a bunch of swordtails, guppies and the hoplos. Some of the other fish died due to odd accidents such as when I put a frozen bloodworm cube and the fish died when touching them... they just flipped to the side and died next to the icy cube.

Or when I fed the bettas catfish pellets and a pair of them got so inflated that they died... then the other pair that died when mom boiled some water and told me to mix it... being a kid, I didn't do well at that and boiled the bettas even though I did pour some cold water in it... -.- EPIC FAIL!
My hoplo has been witness to a lot over the past 9 years. He's one of my childhood pets, along with my turtle and my newt. And he finally got everything he needs barely in 2012: a bigger tank to exercise in, a filter, fish that he seems to socialize with and play with (including guarding the fry) and more food choices.
If only I had known what I got into when accepting my friend's fish tank...

But at least it ended well. Somewhat.