I don't agree with that. If the openside goes down, you play with a 6/8 there and adjust your game plan accordingly. The Brumbies have managed to survive without a 7 their whole season without too much trouble.
You're better off having your 4th best loose forward on the park and playing accordingly, than you're 2nd best openside if these are two different players.
Also the potential 1 or 2 pilfers that the openside would get are outweighed by the potential 1 or 2, line outs/scrums lost due to a compromised set piece.
You can budget 80 minutes from Hooper, you can't budget 80 minutes from both locks. So one is on the bench.
So the other forward reserve comes down to picking someone to cover more positions (say 6, 8 and at a pinch 7) or the player who is a specialist seven?
If Palu is selected, then Higgers, if Higgers starts, then McCalman.
If you really want to open the game up, you pick McCalman as reserve lock (with Fardy) & Higgers