This is the line of thinking I seem to be converging on. When 6ARC came out it seemed like a substantial on paper improvement to the Grendel. However, with a little more experience shooting long range, my feeling is that if I really wanted to shoot longer than the max range of the Grendel, neither cartridge would be sufficient. If I'm shooting 700M+ I am investing a lot of time into planning and setup in finding a spot, setting up targets, driving down to check targets, packing everything up. If I'm going to do all that I want something that hits with authority at that distance and neither of these, or any other, intermediate cartridges seem to accomplish that.
For both of these cartridges that distance is well beyond their effective hunting distance, and the heavier bullets of the Grendel likely have about the same effective hunting range as the lighter faster bullets of the ARC.
That leaves the ARC with one big on paper advantage: flatter trajectory and less wind drift at moderate distances, including the longer end of potential hunting distances. That is certainly a good thing, but I don't think its that big a deal, and I actually really like how the Grendel holds match up pretty close to .308 for training crossover.
The big advantage I see to the Grendel is ammo selection and availability. Ammo is cheaper than 6ARC right now, though the difference with match ammo is pretty small. If the war in Ukraine ends I imagine we will see cheap Wolf again and PSA is supposed to be coming out with 6.5 Grendel ammo at some point. If PSA does that and it is priced similar to their other offerings, that will be a huge boon to the versatility of a Grendel compared to an ARC. I could see them coming out with 6ARC ammo as well but probably just match loads and not cheaper FMJ, and not until after they are producing Grendel.
