Like you said, it's a fact that the capacity of the REM de l'Est trains proposed by CDPQi was very low. At 390 passagers per 40-meter train, that was making a "supposed maximum" of 5850 passengers per hour and direction, and per branch.
With a daily ridership of 133 000 passagers, the REM de l'Est numbers were two times the SRB Pie-IX daily ridership, also two times and a half the Toronto streetcar Queen line. So yes, the REM de l'est has a better capacity but it is the same as saying two trams or two BRTs could make it. The two downsides of thoses modes are:
1- They aren't as fast as the REM
2- They require more trains (or buses) than the REM.
Also, outdoor trains with the cold winters of Montreal don't make a good mix. I guess we'll have some bad surprises with the REM A next winter. We've already had one with the train tests and the cables not well connected due to the cold & ice.
https://www.lapresse.ca/actualites/grand-montreal/2023-01-11/tests-du-rem-de-la-rive-sud/panne-d-une-voiture-poussee-a-sa-limite-dans-le-verglas.php