That was also the plan with Parc and the St.Jerome line (which on weekends and non-peak does terminate at Parc) but people complained and the route to Lucien Station was retained.
And I think will happen the same with Mascouche. People don't like transfers and the majority of Mascouchians want to go downtown.
The difference between this situation and the St-Jérôme line is that Parc is actually a through station, which allows trains to continue to downtown without much hassle. Côte-de-Liesse is a terminal station, so to continue to Gare Centrale, trains would first need to back up to Ahuntsic to switch tracks there. There is absolutely no way the RTM would choose to do that.
Prior to the tunnel's closure, peak-period trains took 16 minutes to get from Ahuntsic to Gare Centrale, with one intermediate stop. Now with the big detour they take 54 minutes, non-stop. In comparison, the future route via Côte-de-Liesse and the REM will take about 16 to 18 minutes:
- 5 minutes in the train between Ahuntsic and Côte-de-Liesse,
- 0.5 minutes to change platforms (only applies southbound, given that northbound is cross-platform from REM to Exo),
- 0 to 2.5 minutes waiting for the REM train
- 10 minutes in the train to Gare Centrale
Currently with the big detour, the end-to-end travel time is 1h45. If there are 4 trains on the line, that translates to service every 60 minutes in both directions* (assuming a 15-minute turnaround time).
With trains turning back at Côte-de-Liesse, the end-to-end travel time would be 55 minutes, which translates to service every 35 minutes in both directions with those same 4 trainsets.
*Yes, I realize that in practice the peak-period service is often uni-directional, but this comparison is just an illustration of the relative productivities of the trainsets in the two scenarios.
Do you honestly believe that people are so put-off by a transfer (which is cross-platform in one direction) that they would prefer a trip which takes 36 minutes longer, and is barely half the frequency?
References:
Extract of Exo5 schedule, 21 May 2019:
Extract of Exo5 schedule, 22 June 2020:
Service summary for Côte-de-Liesse station, from REM website: