I would favor such paths if they're like some of the ones we have in So.Cal. which let you go full speed in any tailwind (I once did 5 miles in 9 minutes on our local 40-mile-long one), they get swept regularly, you can go an hour at a time with no stops, etc.. I'm afraid however that a network of paths out in the boonies would be poorly designed, constructed, and maintained, and could attract vandalism and other crime, and there probably wouldn't be any patrolling. Roads already have the land and they need a shoulder anyway. If the shoulder is paved, a cyclist can ride with a reasonable separation from the cars. We don't need 100 feet of separation.
I've been on paths however that were supposed to be a beautiful, safe way to be separated from the cars, and the curves were too sharp to maintain normal speeds, and the grade instantly went from uphill to downhill and back. Another problem was that the paving was much thinner than normal roads, and roots would grow under the path and push it up, and break it, making terrible bumps.
The only time I feel a real danger on the road is when I'm on the narrowest, winding mountain roads and there are RVs and boat trailers whose drivers don't realize how far they stick out, and that just because the front fender will clear you doesn't mean the trailer will. On a recent trip, I had to be constantly watching the mirror and asking drivers for more room, or even motioning for them not to pass at all until I get past an extra-narrow curve or cut in the rock. There the problem is only in climbing, not decending, because of the speed difference, so the widening would only need to be on the uphill side in that case.