The fact that TM is over capacity on some routes i...

The fact that TM is over capacity on some routes is also a good sign, that people want to use it.

A metro line could certainly reduce TM congestion - but it would cost far more than creating equal capacity with more TM lines.

One possible advantage of a subway, however, is that subways appear to encourage densification in urban development, while for BRT the evidence I've seen is mixed.