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 busiest core routes should be served with light rail, allowing an efficient high-frequency service for the most common journeys, and most parts of a city should ideally have some kind of connection to that rail system within a kilometre or two. But you can’t just put rails and stations literally everywhere, so buses (or trolleybuses with batteries if you’re so inclined) remain useful for less common routes, gaps between stations, the neighbouring areas of rail routes or last-mile connections from light rail to within a short walk of a person’s final destination.
Buses are also necessary as a fallback during maintenance or unforeseen closures on the rail network. Even if it’s just a temporary station closure, that one station will likely be the only one in walking distance for quite a few people (especially if we’re talking about an interurban network where a small, outlying town or village might only have one station connecting it to the rest of its metro area), whereas that same area could have several bus stops, giving pretty much everyone there a way to continue getting around, perhaps even to get a bus to neighbouring stations.
And bus routes don’t change that infrequently. Certainly, not infrequently enough that you’d want to tie them to placing or removing fixed infrastructure like tracks or wires. Diversions also happen sometimes. All of this isn’t to argue against light rail, but to argue for a comprehensive multi-modal vision of public transport. Let passengers use the right combination of services for their particular journey’s needs.