That's not a mainstream forum, that's the new MFK, where discussions exactly like that one take place on a daily basis. This site has shifted from old school fishkeepers with true monster tanks, and many years of real world hands on experience, to the new kids on the block, with their new tank that mom & dad just bought them. In many cases these same kids are now giving out advice on the forums, with thousands of posts behind their names in less than 1 year of keeping a fish, any fish, alive in captivity. What a farce.
I've been in this game & studying fish behaviour for as long as you have Miguel, perhaps longer, and what you stated in a matter of fact manner is nothing more than a personal opinion that is wide open to debate.
Not all fish prefer to be around other fish, in the wild, or in captivity. Many species (and individuals within each species) outside of spawning season, prefer to live a life of solitaire. Yes, when we attempt to keep some of these species in captivity we must cram many fish into the glass box to alleviate aggression issues, yet if we stocked as in nature there wouldn't be any aggression to begin with. Funny how that works. Also, many old bull males, lose their interest to spawn, and become killers of females in captivity. (which is why all male tanks like your mixed amphilophus tank "sometimes" work)
The fish that I keep in solo tanks are flying solo for a reason.
That's one take on the situation, here's another ....... 9 times out of 10 that poor sod that didn't spend the time or energy to properly RESEARCH in advance get rid of their "monster" flavour of the month & replace it with a new "monster" when they get bored of the old one. This is why one seldoms ever sees updates on umbee, or dovii, when they are as thick as my arm & 24" in length. Those enthusiastic poor sods got rid of them long before they reached 20".
I personally don't have a problem with heavily stocked tanks when maintained by those who know how to properly care for the fish in that type of set up.
What I do have a problem with is people who purposely set out to keep fish that due to not just their overall size/shape, but also their energy level (as in swimming space required) are knowingly (as in knowing better) kept in cramped quarters.
Perhaps none of us will ever match "fish paradise", but suggesting to a fellow member that it's ok to keep some catfish that can reach a foot long, a pleco that has the potential to grow over a foot long, and an Oscar in a 55 gallon is IMHO seriously messed up. If that's the quality of advice that this forum has come to, perhaps it's time for me to move on.