Even if you think what you would say is obvious, please add. This is genuinely something I think makes sense regarding local bus routes given the longevity of light rail and how infrequently routes change, but I also suffer from confirmation bias, so I’m hoping for reasons this would be a terrible idea but obviously would prefer reasons it would be an even more amazing idea than I thought.
That’s like saying a ship is more expensive than a car. It depends.
A tram is not „infinitely“ (what absurd statement is that anyway) more expensive than a bus.
Construction cost is not everything, and they’re not even that much higher, you also need to consider service life (much longer with trains), energy cost per passenger mile (much lower with trains thanks to the lower resistance), etc.
What is best is always depending on the specific circumstances.
The biggest limitation of buses is capacity, and a highly used tram is cheaper per passenger mile than a bus. Try replacing the S-Bahn in Berlin with BRT, see how far that gets you. You’d probably need to bulldoze a new highway… speaking of which:
Germany is actually hellbent on building a highway right through its capital Berlin, which currently clocks in at 700 milion € for 3.2 km. I expect the whole thing to end at ~2 bn € for ~7 km.
So I think the costs of public transport are really not the issue people should be focusing on.