You may be confused by the title of the thread, which I was as well. I don't think we're talking SLE-Hartford Line through service, but rather, SLE thru trains to points on the New Haven Line and Hartford Line thru trains to points on the New Haven Line. Stamford is a big employment draw in the SW CT region and may in fact attract some from points on the Hartford Line. Maybe try some rush hour expresses, like SLE to Stamford used to do, for Hartford to Stamford, or something like that. It may also labor issues that prevent it from happening. Obviously there would have to be a crew change at NHV. Not sure the status of the leased MBTA cars as far as whether they are permitted west of NHV.
Now, if Metro North somehow was able to operate the three systems (NHL, SLE, Hartford Line), then it probably would've been done by now. They would have to bid on/get the contract to run the service, if they even wanted to.
Then there's the issue of track space as well. You'd probably want those thru trains to stop at Bridgeport, and that's tougher right now given its only got side platforms. Are we at the limit as far as what can be sent over the draws west of NHV? The NHL I believe is close to/if not back to pre-COVID service levels, so could more trains be squeezed in?
Then there's the question of more Amtrak regionals. Well, that's all well and good heading south, but if you've got a regional heading to the Hartford Line from, say, Virginia, then there's more of a chance of it being late. Same goes for the existing 400-series shuttles that connect from NEC mainline trains. You'd be better off having CTRail take over the shuttle spots, keep the Vermonter and a couple regionals for thru service from SPG to NYP & south, and give the rest over to CTRail. Or, have the Hartford Line regionals go no further south than NYP. But Amtrak's theory for trains from the north is to pretty much run them through at least to WAS.