But to the following aircraft, won't the wake still be reached in the same time, for a given spacing, regardless of the wind?
I'd have thought that the distance between the preceding aircraft and the wake vortex it generates would be a constant, regardless of headwind strength.
You seem to agree:
In simple terms, the airborne aircraft and the vortices don't "know" there is such a thing as wind.
In that case, for a given spacing, the following aircraft
would take longer to reach the preceding aircraft's vortex in a stronger headwind.