There's already have a system in place that scales stats (substats, defence values, block rate/str. Not main stats though) to a hidden level modifier which quells, and to some extend "resets" them at expansion launches, which I think keeps stats from blowing up too much quite well. If you look at for example tanks final stats (dps & hp) at the end of each expansion, those values don't grow all that much compared to other MMOs.

Stat squish may come one day but it's most likely not next expansion or the one after.