If we were talking about leisure cycling on a sunny day, then that might be true, but we aren’t. People need cycling and walking routes that go past the places they need to get to. Very few of those destinations are on canal towpaths or old railway lines; they are on the road network.
Off-road routes are also inaccessible to most people for much of the time because they are almost always remote, unlit, unpopulated and unsafe. People don’t want to have to cycle home from work on deserted paths in the dark even if they are pleasant enough places in the daytime.