Even then, what's the point of extending the Interstate designation a few miles west of the current terminus?
I still don't think that US 36 across Missouri needs to be converted into a full-freeway. There are bypasses of the towns, grade-separated interchanges at major routes, and the at-grade intersections at minor routes aren't problematic. When I drove the route, it was very lightly traveled, and even a doubling of traffic volumes wouldn't create issues sufficient to spending big bucks to upgrade it to a full freeway.
Perhaps states should come up with some sort of new uniform route marker that is shield-shaped, but does not duplicate the existing Interstate marker, and color it red and blue and post it along routes it wants to have designated as an interstate (or for surface expressways like US 36). For Missouri, it would be a fifth class of highways (lettered supplemental, state-shaped state route, US and Interstate); for Kentucky, it would be a fourth class (circle/oval state, US and Interstate). Either let AASHTO administer it, or if it involves just two bordering states, let them figure it out.
Missouri could post US 36 as this class of highway and number it 72, and co-sign it with US 36 and CKC 110.