Would also largely keep intact the current I-64 west/east to I-81 north connections.
Yes, but then it would eliminate the more important I-64 West to I-81 South connection. That connection today provides access from the I-81 corridor heading to/from the southwest to I-64 bound to Richmond and Hampton Roads.
With I-64 intersecting I-81 the way it does in both places today, it maximizes the connections between I-81 and I-64, only leaving out the least important I-64 East to I-81 South movement that is covered by US-220.
If I-64 had been put on the southern route, it would still intersect with I-81, somewhere between Troutville and Lexington. There may not have been as much overlap (or none, if it followed the U.S. 220 corridor to Clifton Forge) but it would eliminate the current situation for anyone from Roanoke and beyond of having to drive north to Staunton, then southeast to Richmond.
It would intersect with I-81, but it would become out of the way for traffic from I-64 East to I-81 North, eliminating the ability to adequately provide that connection.
The current northern routing would’ve only been reduced by 10 or 15 minutes if it was built along US-220 and US-460, and would’ve added significant new construction mileage.
I-64 still provides the most direct, fastest connection from the I-81 corridor and points southwest into Tennessee along I-40 and further south from I-75 bound to Richmond and Hampton Roads.
US-58 and US-460 provide alternate arterial corridors that provide shorter mileage, but are slower overall with lower speed limits, traffic signals, developed areas, etc despite having 4 lanes and bypassing most towns.