Actually, I thought this was the clever part. If you pay attention, they created a lot of other problems as a side effect.
It wraps up all the existing plots, and introduces new ones. There's a lot of hints on what went wrong with time travel, and if you read up on Marvel lore, there are plenty of villains who are able to maneuver time travel better than the heroes.
And it created a wonderful narrative device to revisit scenes and references from several other filmes. Marvel movies never positioned themselves to be hard scifi.
It wraps up all the existing plots, and introduces new ones. There's a lot of hints on what went wrong with time travel, and if you read up on Marvel lore, there are plenty of villains who are able to maneuver time travel better than the heroes.