Pokémon Go, being a collection-based game, could have easily been ruined by trading. If trading is easy, then why would anyone bother to go out and walk to hunt for rare stuff when they could sit on their couch and search the trade options instead? This would both mess with Go's economy and run counter to Niantic's vision of the game. Not to mention, you would also have botters and spoofers handing out free rare Pokémon and/or selling them, destroying the economy of Go even further. Personally, I didn't even want trading implemented, as I was afraid of it ruining the game, but I am actually quite pleased with the way this system turned out. It's not perfect, and can still be exploited, but it's better than I thought things would end up.
That all being said, I still think that regionals need to be done away with, or at the very least, have events like Farfetch'd for the other regionals, even with trading introduced already.