A path is too sterile, i.e. no hills, long distance views (as trails are typically surrounded by trees), etc. I do like the quiet and relative safety. And having toured in the Netherlands, I can agree with the posts about the seemingly random direction the paths take.
However, I really doubt our country has the money for a 12' wide bike path across the entire country. It would cost at minimum close to $100k per mile without easement buyouts. I have no idea how it can be that high but those are the minimums I have heard tossed around. That is almost $300 Billion for a 3,000 mile route. But hey, the way Congress is throwing/wasting money around, why not?
Instead, I would prefer a minimum 2' shoulder on all state highways and a 3' shoulder if a higher traffic count. Also, if all states would develop decent bike research (like what Missouri and Wisconsin does), that would be a big help.
Just my 3 cents (inflation and all).