None of this would happen in the real world, but we are not talking about a real world. We are talking about a comic book world, where all of those things have worked. Humanity was able to protect itself just fine without superheroes, but now humanity is entirely reliant upon them.
Superheroes also have a much worse track record than normal humans - the Skrulls have attacked Earth regularly for 50 years and, yes, have been stopped each time by superheroes, but they soon attack again. But when a normal human defeated an alien race, it was rare if they ever returned. Superheroes merely give aliens the impression that we NEED superheroes to defend us.
In the end, in Riff's world, humanity will do just fine without superhumans (especially when you consider that with superheroes came supervillains too). But with superhumans, humanity becomes obsolete and starts to think that they can't do any of the things that they were once capable of, unless they too are superhuman.