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.
The problem with buses is that most of the fleets are still running on fossil fuels. Buses also produce a shocking amount of waste in used tires. 
Yes but depending on utilization and the size of the city they can have a much lesser impact compared to the creation of a light rail network.