From my experience (in my town...) a well-designed, well-routed MUP might start out with very little traffic, but it grows each year. When I started cycling in 2005, the MUPs here were largely empty after the summer. Lately, the MUPs are are being plowed in the winter because people are using them....

But using a separated bicycle infrastructure for your whole plan isn't likely to work either. You need some combination that includes some ability for people to uses routes on city streets.

My feeling is that in Des Moines (and this might not be the case where you live...) we could stop expanding MUPs and start looking at concepts like "bicycle superhighways"... basically designated routes through low-car-traffic area. Instead our city seems intent on going to bike lanes... which are great except that the majority of them are adjacent to parked cars.