Concur. It'd be fairly easy to put US 460 on the Mountain Parkway. As for the broader question of what the rationale would be, there are a number of reasons. A) that subset of the driving population who more easily navigates via route numbers, B) numbers are easier (usually, maybe not for everyone) to remember than names, C) easier for inventory and database purposes, which is likely why KYTC has hidden route numbers for the parkways already.
I think an even better rationale is that US 460 is not really a major through route west of Salyersville. It's basically a road that serves county seats and local traffic, even though Kentucky's rebuilding it in increments. It's not a key interstate (as opposed to Interstate) link. No one in Salyersville is going to use US 460 to get to the central part of the state.
So, right there, one doesn't even have to reroute highway routes onto the parkways, because they already have numbers. Simply sign KY 402 along the Mountain Parkway.
It was interesting to watch Kentucky's progression on what to do with the route once the tolls came off. (It was the first toll road that wasn't already signed as a numbered route to become free). Kentucky first put KY 114 on the route, although it was never signed. Then came KY 402, which was only signed when Exit 33 at Slade was rebuilt. Those signs only lasted a couple of months before they were replaced with the standard old circular blue and green markers, and I regret not getting a photo. The 9000-series came later. I think KY 402 got reassigned somewhere out in the far western part of the state.
Occasionally some maps or mapping software will still show the route as KY 402 or KY 114.
