When does light rail ever make more sense than either BRT or a metro system for any reason other than construction costs?

I've often heard people say it is a mistake to treat light rail as just an inferior alternative to a metro system as there are niches for which light rail is more suited than a metro system, but I've never heard a compelling example of this that doesn't come down to initial investment cost. It seems to me that there is never an instance where light rail is not either overkill for what could be BRT or underkill for what could be a metro system, since the major advantage of light rail over BRT is capacity but by the time a bus corridor has the capacity to justify a light rail system it also has the capacity to justify a metro system, and the grade-separation of a metro system has no major operational disadvantages over a typical light rail configuration. Once again, the only real advantage of light rail seems to be that you can get capacity approaching a metro system but at lower initial investment cost, which is not an example of light rail being more well suited to something, that's just getting what you pay for. I'm not saying there are not plenty of valid scenarios where light rail might be the best economically viable option, but that doesn't make not an inferior alternative to a metro system, it just makes it a *more affordable* inferior alternative to a metro system.