Yeah my last irreparable flat on my Outlander (full time AWD) resulted in a full 4 tire set. AWD has it's perks but not without a few trade offs. At least when that happened the remaining tread was getting low I was probably going to get at most one more summer out of them.Good thing that didn't happen to a Subaru or you would replace all 4 tires.
I have gotten one irreparable flat in the truck too but I had less than 5000km on long trails so there wasn't much of a tread wear difference after replacing it.
I have also heard of tire shaving to bring the new tire into check with older ones, but I have yet to meet someone with AWD admitting to doing it.