Echoing some of the thoughts already shared with my overall take...
If there were such an Interstate MUP. Many commuters, tourers, road cyclists, fitness riders, etc etc would avoid using it anyway for several reasons.
Faster speeds are capable on the road. Less pedestrian traffic or obstacles on the road, More direct path to destination on the road, Better and more roadside destinations on the roads. Many cyclist have overcome anxieties about sharing the road enough to prefer it over MUPs.
Project Interstate MUP seems like it would only be catering to one type of cyclist. A touring cyclist who doesn't like roads. Granted limited stretches would be used by locals.
Creating, or piecing together a vast network of linked paths from one end of the country to the other for a minority demographic doesn't seem realistic.
Lots of folks just prefer roads.
Being a person who actually does currently prefer MUPs myself, I'm all for the creation of more quality paths. More so for paths longer than 10 miles, or connections to neighboring paths. I enjoy the care free ride, don't mind the pedestrians, and do push myself on distances. I'm just casual I guess. Though have contemplated a future of doing some bikepacking.
With that said, I can get behind smaller scale en devours like paths that can take you across a good amount of the state. Though don't deem a nationwide network to be necessary.
Bringing things into scope.
I get that you (OP) wish for extended safe terrain, but ST RT MUP is a pipe dream. Your only real options to come close to satisfaction...
Accept and appreciate what you have nearby, or drive further out to newer/longer/better paths.
Lobby with local governments and parks depots on more or extended local paths.
or,
Get accustomed to sharing the roads.
As for as the point of view of "the more paths there are, the more we're expected to be off the road" discussion... I can't agree with that side point. As much as I would personally lobby for more paths, I'd equally lobby for better shared road conditions. Perpetuating the safe shared road possibilities. I doubt long established laws that grant cyclist the right to ride on a road would be changed if given a nearby multipurpose path.