besides remember how the US government freaked in the JLU cartoon when they found out that their satellite had been weaponized with alien hardware? The Romans spent 200 years arguing down the idea of a fire department because they didn't want an organized and trained body of men inside the city limits who might at any point be repurposed for political measures.
TWO HUNDRED YEARS!
If the Avengers are going to turn into a massive army... Don't you think that the US government is going to be mildly concerned and try something stupid again like outlawing super powers, reinstating the capekillers or civilian oversight?
Think about the similarities between Avengers Mansion and Waco.
Everything could go so wrong so quickly.