it's something the valley line will NEVER achieve. H
I don't know where you got that info, the Valley Line could very well achieve 90 second frequency, provided we have the necessary amount of cars. Plenty of cities out there with similar low-floor systems that have such frequencies, including cities in places you'd never expect any kind of efficiency on ANYTHING (looking at you, Rio).
$2billion for a line with limited capacity
Any transit mode will have limited capacity. Some limits are higher than others, but there's not a single limitless transportation mode in existence. Now, as for how they can address capacity in the future, there are a few considerations:
1 - they can run trains of up to 3 cars, if I am not mistaken (could be 4). That in itself would increase capacity, as it's just a matter of acquiring more cars, with 5-min frequencies.
2 - they can increase frequencies. 3 minutes should be easily achievable. Again, a matter of acquiring more cars.
3 - It's hardly conceivable that any lines in Edmonton will reach the kind of capacity issue that would be concerning. Worst (or best) case scenario, the CMA population will likely peak at 3.5-4M in a few decades, and by them we'll very likely have more lines, and we definitely still have space to improve capacity on the existing ones.
If you think of the Valley Line, for example, with an increase to 3 minutes frequency and 3-car trains, the increase would be as follows:
Current capacity is 12 trains/hr and 2 cars/train, each way, for a total of 24 cars/hr/direction. With 3-car trains and 3-min frequencies, you'd have 20 trains/hr and a total of 60 cars/hr/direction, and increase of 150% in capacity.
4 - the bulk of our population growth (and by default, demand) will very likely happen around the higher capacity high-floor lines, either by design of by choice. That is on top of the fact that Edmonton is a very de-centralized city, so we'll hardly ever see the kind of ridership going towards a single point (usually DT) that would cause massive issues.
Even if we manage to increase the percentage of the population that uses transit, aided by the population growth, unless we achieve Europeans level of users, our capacity is not likely to be an issue.