The fixed-route signals to people that your transit investment is there to stay, and so they can bet on it still being there in many years' time.

This is a myth, as explained at http://humantransit.org . The truth is that because a bus route is flexible, the transit agency can make slight changes to a low-performing route to try to save it. Yes this might mean that the bus won't stop at location X anymore, but it is a way to preserve service to Y, Z, A, B, C, etc. that are also along the route.

Rail does not offer that same sort of flexibility, so if there is a low-performing route it cannot be "saved" by a slight reroute, so if funding runs out the only option is to cancel the route.

But the rail is still in the road!