I'm not suggesting that the money from the savings of choosing this technology be put in some sort of savings account to save for another relief line a generation from now. I'm simply suggesting that the opportunity cost of spending so much money to future proof for an event that may not occur for several generations unduly limits our ability to improve the situation today.
The reality is that with the funds we have we can have a line that goes from Liberty Village to the Science Centre, with marginally reduced capacities, but still enough to service decades of ridership growth - or we can have a line that goes from Osgoode to, maybe, if you are lucky, Thorncliffe Park. Then you have to wait another decade for more funds to turn up to build the rest of the thing. But hey - you would have 3 more trains worth of capacity every hour for the event that, 30 to 50 years from now, it may go over capacity.
Is that a better alternative than having the line now, for the whole route? In ten years we can then have the line extended to Sheppard, instead of Eglinton because of it.
Is the benefit of having that marginal additional capacity of 4,000pphd which won't be needed for at least a generation, if not more, worth more than having the additional service to LV and the Science Centre today? that's the comparison that has to be made. The timelines for when that additional capacity is required is simply too far off, with too many unknowns, for it to be an appropriate call.
The opportunity cost is simply too great.
If that extra capacity is indeed required 40 years from now, it likely will only be "good" for a few additional years anyways before even the TRs go over capacity. In which case you need the relief line for the relief line anyway. You may have staved off a major investment in better transit coverage by a decade, but in the process you have reduced transit coverage for a generation before hand.