The Russo said Hulk physical state is permanent https://www.comicbookmovie.com/avengers/avengers_endgame/avengers-endgame-co-director-joe-russo-reveals-the-hulks-new-status-in-the-mcu-moving-forward-a168221
I hope they're kidding
Hard to imagine, but undoing the snap is a huge undertaking. Losing one arm in pursuit of that goal doesn't seem like that bad a price to pay.
Remember, too, that this was probably the arc Ruffalo wanted to see for Banner/Hulk. There's a sense of permanence in his final condition - like this is where he ends up and it's OK. All six of the original Avengers have now moved on.