the problem i see with all your theories that if there werent a bunch of ignorant people killing fish through lack of knowledge, that fish prices would skyrocket and nobody would buy fish, is as follows.
if pet stores told people the truth, that you cannot keep these two fish together, that you need to cycle your tank before adding fish, that this fish wont fit in the sized tank you have, etc. then people would come to trust that store for not lying to them and making them waste money, and would probably be willing to pay a little more to go to that store they trust.
if they trained the employees to tell people, "i'm sorry, but you really should have a 55 gallon tank for an oscar because they can get up to a foot long. what about this fish instead....." or even "We have a sale on this great 55 gallon tank"
or told people "If you want to set up a new tank, its a good idea to cycle it with ammonia first. its much safer for the fish and you'll be less likely to lose them. then when your done cycling, we'd be happy to help you pick out fish!"
it builds trust in that pet store, a knowledge that they're not just after money, but care about you, the customer, as well as the companion animals they're selling, and people would be more likely to shop there.