Or make the subway transfer suck less, fix the Braintree branch, and integrate fares and then CR->RL becomes actually reasonable.
The T would have to make up a lot of ground to make this feasible. The current schedules put CR at about 23 minutes from Braintree to South Station.
In 2016, the Red Braintree -> South Station journey took about 28 minutes (maybe closer to 30 during peak).
Yesterday, it took an average of 43 minutes.
(With literally four trains within each hour.)
If brought back to 2016 journey times, frequencies, and reliability, supplementary Old Colony shuttles transferring at Braintree might well be viable. The transfer is still a bit hostile, but not unworkable.
But that's a lot of ground to make up between 2016 and today.
(In some alternate universe, the Old Colony or some vestige thereof kept all lines continuously operating, terminating at a rapid transit transfer station at Quincy Adams. Like the Mattapan Line, but, you know, bigger. The southern Red Line was, after all, designed as a
trunk line for a distributed system of branches out across southeastern Mass.)