Many people are rightfully concerned that the % reduction in cost will be smaller than the % reduction in capacity.
Say, we save 15% off the cost, but lose 35% of the potential capacity by using smaller and narrower trains. No problem on the opening day; if the line is designed for 34,000 or even just 29,000 pphpd, it will easily handle the opening-day demand of 15,000-20,000.
But 15 years later, the ridership of OL will hit the capacity limit, and we will be back to today's situation of not being able to serve all peak-hour demand into downtown.
While if we had that extra 35% of capacity, OL would remain sufficient for 25-30 years after opening, instead of just 15 years.
People need to avoid equating underground to capacity - it muddles the conversation (and I don't doubt for one moment that the local councillor cared more about the former than the latter). In any event at issue with OL is that we know the current termini won't be the final - any projection should take into account reasonable extension scenarios. Flexibility comes with cost - so let's pay for it when we had no hesitation paying for it where it isn't needed.