If i was Grundy, I'd look to move one. He's way too good to be a backup ruck.
However, Gawn has a lot of miles on the clock. And a large part of the thinking regards bringing Grundy in was it could extend Maxy's career. He can't shoulder ruck duties on his own. We'll accelerate the end of his career if we revert to that.
It's a big conundrum to figure through before the trade period. Personally, I reckon we should hold Grundy for another year and see how things play out.