Toronto is the case study of what you get with this line of thinking... rail lines are meant to last up to 50 years, whereas buses maybe 10. We have buses criss crossing all over the city and a chronic shortage in capacity. The connections to the existing lines are fine but then causes massive bottlenecks in the system. When the plans were made the city didn't "need" this capacity, but then there was massive growth and now we're finally playing catchup and are soon to move beyond a 1980s transit system.