Capacity is not, in ANY WAY, based on technology used but rather on station sizes and frequency levels.
Sorry, but that statement is purely false. Every technology has limitations with regards to the size of station, the frequency the service can obtain, and the size of vehicles. Sure, you can link up a bunch of light rail vehicles, but at what point does it become cost prohibitive, or increase the cost of construction? I can theoretically make a 6 LRV long train that has a similar capacity to a Toronto Rocket, but I'm going to provide more wear on the pantographs, require platforms that are 42 meters longer than a subway platform, spend more on rolling stock and maintenance.
But what about running said service with smaller trains but with much higher frequencies? Well that creates its own set of challenges in it of itself. Sure, automation can reduce bunching, but at the same time, it severely decreases speed. Think of fluid dynamics and Bernoulli's law, if water is under pressure, the velocity is a lot lower, however, if it's pressure is a lot lower, the velocity is a lot higher (for ideal systems). The same principle can be extended to trains: If you have more trains running on a line (an increase in pressure since the space between trains is a lot less), then the ideal speeds the trains can run at will reduce, and if you decrease the number of trains on the line, the space between trains increases, meaning speeds can increase. If you sacrifice speed for frequency, there comes a point where your service isn't rapid any more, and doesn't provide enough of a justification for people to use it.
The technology used has everything to do with these complications. With heavy rail, specifically the subway technology we use in the system to this day, we have wider trains that have the potential to run at frequencies of every 90 seconds. You can only make your platforms so long, so if you decrease the width of the trains, you're taking away potential capacity. If you're trying to cut costs, increasing the diameter of the tunnel will be significantly less expensive than building a station with a longer platform, so cutting the length of trains can still provide decent capacity while cutting costs. Of course, this is a dumb move to make on the Ontario line due to capacity constraints.
There's also another problem with the frequency argument, it doesn't account for huge surges in demand. This is going to be a huge problem at all the transfer stations, especially Pape. You can see how these demand surges affect train capacity on the Yonge & Sheppard Lines at Sheppard Yonge station. If you have a subway bringing in a huge number of passengers at Pape, and it dumps say, 800 passengers off there to take the Ontario line, some passengers may have to wait for up to 3 trains (probably more since there will be traffic from Science Centre) in order to make the transfer. It also doesn't help keeping the line on time because assuming the line is running at a maximum train capacity, the trains behind it will have to wait for all passengers to board the trains currently at Pape. This is enough to sway people away from using the Ontario line as a potential service.
Sure, if you were to build the Ontario line as the preexisting relief line with the technology provided, sure, you might be able to get away with it, but once you start adding passengers from Sheppard and beyond, the line will be a sitting duck, a huge mistake for the city that should have built a subway line instead. This line is supposed to compete with the Yonge subway, which sees 30+ PPHPD north of Bloor, and assuming ridership continues to grow throughout the city, the Ontario line will not be able to keep up with the passenger growth north of Bloor, and will eventually become just as crowded as the Yonge line is today.
