Ya, sure, parrots have established themselves in various places. The feral flocks are thriving and doing great! However, if there is no feral flock to join, a bird has a much smaller chance of survival. Even if they could join a feral flock, doesn't mean they'll survive. Apparently, several budgies have joined the conure flock in San Francisco.... all that have joined are no more.
Many of the parrots who get lost outside? Proably many of them are clipped... so they are in greater danger from predators... dogs... cats... coyotes... foxes... hawks... even if they aren't clipped, a bird who does not receive enough exercise may still not have enough strength and stamina to get away from some of those predators!
There are people who are trying to re-introduce certain parrot species back into the wild... and without a starting colony already there, these birds often struggle to survive... they struggle to find food, and to keep away from predators. The reintroductions? Failures. Living out in the wild isn't all that it's cracked up to be...
I stopped buying birds in 2004 because people kept giving me birds... the only birds I ever purchased were budgies... (and technically a cockatiel, too) but I've had budgies, bourkes, a turq, a lovebird, cockatiels, an african ringneck, 4 different species of conures... I've had 3 separate people offer me a cockatoo or want me to take a cockatoo home. So far, I've turned them all down. (and all have since found homes)
Since I've had so many second hand, plus birds, and my 'nest' is feeling a bit empty (older birds = health issues = them passing + cancer is a b____), I'm considering getting another bird, but I haven't decided on what, yet. In fact, I'm even considering a hand raised bird. With the exception of Casey, the cockatiel that I technically bought, I've never had the pleasure of a young, hand raised bird. Likewise, I wouldn't be against a parent raised bird of a certain species.
At least the bird hobby isn't nearly as bad as the fish hobby is.... if you want to talk about atrocious care and handling.