The reason why the CTRail schedule is on a slow ramp-up is because the Springfield layover facility at Armory Jct. hasn't been built yet. That's the one funding piece for the Hartford Line that MassDOT was on the hook for as the largest payee, and they've so far refused to proceed with it without fed grant help. CDOT physically can't do 17 round trips without it, because Springfield Union and the old AMTK mail yard by the junction are too small to juggle a full commuter slate with the Amtrak schedule. They have limited slots to idle, which is why the starter schedule has so many trains running just NHV-HFD.
It'll get done. CDOT's not really ready for the full-blast Springfield schedule anyway with Windsor Depot still needing ADA platforms, Windsor Locks still needing to relocate up the street to a new station location fed by faster airport shuttles, and the bulk of Enfield/Thompsonville station construction beyond basic land clearing still yet to begin. All 3 of those north-of-HFD intermediates lagged the rest of the project by a few years because of late funding releases and are only now full-funded, final-designed, and shovel-ready. And there's still work to do with the North Haven and West Hartford infills, as well as the whole multimodal follow-through of connecting all those stops to new/enhanced last-mile local buses and private business shuttles started with economic development $$$. Hartford Line was always intended to be a slow-cooker like that during its first 5 years until all the connecting pieces were in place to really drive the ridership to an increased schedule. The Year 1 ridership is probably going to be tidy because of all the other moving parts that are still cooking, and that's why they aren't sweating reassignment of the Shuttles yet.
As for MassDOT, they may simply be looking for a way to roll up Armory Layover in a wad with further action on the NNEIRI studies for the Inland Route, since Shuttles being extended to Boston would mean MassDOT could claim some of Amtrak's share of layover space at that joint AMTK-CDOT storage + MOW facility for future Knowledge Corridor commuter rail without needing to pay more to outright expand the yard beyond current plans. They can't punt it off forever, and certainly cannot ever consider homegrown Conn River Line commuter rail without full-funding that facility for AMTK + Hartford Line in advance. Right now just appears they're picking their spots for most opportune timing and advantageous financing to jump in and aren't in any present-day hurry to put a rush on it.