I mean what are we really saying here, that the OL should have been 1 train car longer with 120m platforms?
I understand concerns about capacity, and yes, the OL will likely eventually run over in the distant future. It would likely eventually run over in a slightly more distant future if it has been built with an extra train car or two of length as well though.
The question from a project justification standpoint is more of an accounting game.
Take how much extra cost you incur by adding that capacity (making all platforms 20% longer, introducing more pedestrian circulation space, increasing the size of the yard to store the trains, etc.), multiply it by all possible extensions which will be "stuck" with that capacity, despite it only being needed between East Harbour and Queen, and get your additional capital cost.
Take that cost, when that capacity is not expected to be required for 30-40 years, Then discount it for the period in which it is expected to sit idle and unused. What that rate is, I don't really know, but an extra couple billion sitting unused for decades is going to be very, very expensive once you discount the time that capital is just sitting there, depreciating. Never mind the fact that it would increase operating and maintenance costs, even if you are only running 100m trains and not using the extra 20m of platform space.
Then calculate how much that extra spent capital today delays the need for a third subway line. If we are assuming that the line will grow from 15,000PPHD on opening day to 30k in, say, 2060 (accounting for eventual extensions increasing passenger loads), maybe it gives you an extra 10-20 years of life of the line without needing the third subway line.
Now what's cheaper - spending a couple billion up front in 2022 to delay the third line need from 2060 to, say, 2080, and letting that capital sit dead in the water for 40 years until it's needed, only for it to be 'useful' for 20 years before the third line is needed anyway, or use that capital today on other, better returns on investment (say, other transit improvements that can actually deliver results today), and instead focusing transit capital in 2060 to build that third subway line then, delivering the benefits of a third line 20 years earlier.
Almost nothing in capitalism is built for 100's of years of growth potential. There's a reason for that.