Part of the justification for the MBTA to do the pilot was the constrained parking availability at the nearby Providence and Franklin line stations: Manfield/Sharon/Norfolk/Walpole. The pilot has not attracted riders from any of these stations to Foxboro. Instead, the people who board at Foxboro are probably the same people that parked at the Foxboro town lot on the Manfield line that has a shuttle bus to and from Mansfield (making it a two-seat ride). And by cutting service at Walpole (1000 boarding per day), it has detrimentally affected the commute of many for the benefit for the few (50 daily boardings at Foxboro?), and it has created a parking constraint at Norwood Central that did not exist before.
If they commited to returning service to Walpole and using the double-track expansion to run "local" service to Foxborough and express service to Franklin (bypassing many of the small stops - plimptomville, norwood depot, islington, endicott, readville) that make the trip from Frankling unberably long, then I think the expansion makes sense. Running the trains from foxboro via the fairmont line make sense in terms of saving equipment, but it makes the trip from Foxboro much longer than the trip from any of the surrounding stations.
On the fairmont trains, the ride from Foxboro is over an hour, or equivalent to the same time you can get from PVD station to Boston (which is 20 miles further from Boston, almost twice as far!). It is a 20 min longer rider from Foxboro than from Mansfield and at Manfield you have more trains to choose from as well. Just so we can get 500 free spaces from Bob Kraft? The money would be better spent expanding parking options at existing stations that are parking constrained. There are open parcels that could easily be made into surface lots at Attleboro, Manfield, and Norfolk