You cannot compare GO commuter with RER because they serve totally different demographics.
Commuter rail serves ABLE BODIED worker going to the office and back and that's it. RER is for off peak service where you have many more seniors, disabled persons, cyclists, parents with strollers, students with huge knapsacks, and people carrying their huge shopping bags. This brings the entry/exit loading areas to a crawl as everyone tries to negotiate around these people. Also because RER is not just a one destination service you will get many people who are going short distances and hence will linger at the entry. This is much like subways where there maybe seats available but people will just stand near the exit if they are only going a few stations.
People using RER will be a completely different demographic than those using commuter rail and this should be reflected in the trains that serve them.