> So if [bike] commuting were the only purpose of the road, you would essentially never need to replace a road again.

You're assuming that use is the only thing that harms roads, and that's clearly wrong.

I mentioned tearing up the roads for utility changes.

Another factor is weather, which amplifies the problems caused by utility "tear ups".

It's fairly easy to see these effects; pedestrian-only and bike-and-pedestrian-only "roads" deteriorate.

However, we know also know that (as you mentioned), these roads won't be bike only.

In short, there will be some maintenance savings, but not a lot.