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.
Aarhus is not really a good example of replacing a bus, as it is a rather regional light rail system with a short inner city section. The difficulties they have seen are probably mainly caused by the technical and budgetary choices made during planning.
For Denmark, Odense looks like a better example, that should be successful if they manage to solve the initial challenges, e.g. with noise/vibrations.
As for the longevity that OP is mentioning, the systems in Bergen (Norway) and Tampere (Finland) show how important this is, with huge private investments being made along the lines. A bus line can be gone next year, but rails will stay for at least decades.