For what it's worth, if you don't *need* it now, I would wait. While it's true that it's expected that M2 > M3 upgrade for the MBA should be minimal / just a spec bump, you never know what might end up being released.
Point being, although it's true "you'll always be waiting if you wait", when we're likely this close to a new version, for me, it makes sense to wait. Discounts on existing models likely will remain (or if anything, increase) after the release of the M3 MBA, so don't think there's necessarily any lost savings of not buying now.