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 big benefit of light rail is you can make trains longer than buses, and fit more people. So if your system has outgrown buses, then you should move to rail.
But transit systems should always be trying to maximize frequency, because the more frequently a train or bus comes, the more convenient it is for riders. So if a bus fits 30 people and a train fits 90 and you’re trying to make a decision between providing a bus service every 10 minutes or a train service every 30 minutes, the bus service is the better option.
Different modes work best for different passenger demands, and you should use the right one for the number of passengers you’ve got. Overbuilding is expensive, and if you spend too much building out a network and the don’t have enough for operating expenses then you’ve got to reduce service levels.