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’s not an either/or thing. Buses are great (if they are well funded) and light rail is also great
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.
exactly.
you can’t assess the merits of an integrated transport system by arguing which one one mode of transport betters all others.
some places /routes (at some times) might work best with one option.
but most places / routes will be better served by several types at least at some times of day.
buses are one of the most flexible public transport options, fill gaps in space between high capacity modes, and fill gaps in timetables, and they sometimes fill gaps in affordability usually being cheaper.
give them bus lanes and priority at junctions, and they’re a lot cheaper and more flexible than trams.
i always think that a busy packed bus lane is making the business case for a train, but filling the gap in the meanwhile. and sometimes a train is impractical.
they didn’t only get rid of most of the trams in the uk due to cars wanting more roads. it was also because buses were cheaper and provided much better routes that could flex to travelers needs…