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.
You wouldn't even have to go for the "replacing the S-Bahn" to show how ludicrous a BRT is as a suggestion, unless you're not paying the constructors and drivers a living wage, which is why it makes sense in say Colombia and not in Germany...just think about replacing the M-lines of Berlin tramways with a BRT. It would have to be couple meters wider, would be terribly unreliable and inefficient, not to speak of noisy and bumpy. Now who would want to have that? Not to mention how much the upkeep of two lanes of dedicated BRT costs vs. maintenance of steel on steel rails and catenary. (Most of the time you'd find the latter to be cheaper.) In Helsinki, Finland we are currently waiting for a new tram/light rail option to replace a bus service that should have been a modern tram/light rail line in the first place: https://raidejokeri.info/en/ In the neighbour municipality Vantaa some parties were trying to push for a BRT option but the independent research suggested light rail/tram option to be the best and this is what was chosen: https://www.vantaa.fi/en/housing-and-environment/traffic-and-transport/vantaa-light-rail (they call it light rail but in some ways it's also reasonable to call it a tram)