According to the NEC Commission, Track 6 electrification work for SLE will be underway this fall. That certainly suggests that there is a plan for electrified SLE service beyond Old Saybrook in the future. With relatively low passenger volumes, I wonder if they could handle loading and unloading through a single door with bridge plates on track 6. When I recall taking SLE from New London, I believe track 6 loading would be through a single door as well.
I heard in some unexpected context, I think an ENO center webinar, that M8s *will* be put into service for the Stamford direct trains. That stuck with me because it was, to my ears, the first time a high-ranking DOT official has confirmed a plan to operate the M8s on SLE, rather than just test them. 6 car sets make sense for those trains.
I hope the M8s are fully utilized across SLE. I'd much rather Siemens' limited manufacturing capacity be spent building new trainsets for Amtrak than for SLE, where M8s can feasibly and superiorly operate the service.