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.
It would mean that, for very high volume and consolidated bus routes where adding new busses would actually start to contribute to congestion, a city can then decide upon “upgrading” the entire route to a tram line. Of course this means changing the infrastructure and lane structure on the involved streets (and, sometimes, not only there) and possibly the layout of the stations