If you have some evidence to back this claim up, I will happily change my mind. However,
OpenRailwayMap suggests that half of the route currently has speed limitations of 60 mph and the other half has 40 mph. By my eye, the 2-mile long zig zag from Columbia Road to Morton Street is sharper than anything on the NEC.
View attachment 23130
And again, combining Amtrak's 120 mph trains with Fairmount Line trains,
which average 18 mph with stops across its 9 miles, seems unnecessarily complicated, especially when you have the NEC right there, with 3 (and possibly eventually 4 tracks south of Forest Hills) tracks available, and
where MBTA trains average 41 mph with stops and routinely travel well in excess of 60 mph for long stretches.
Rerouting Franklin Line trains over the Fairmount is a better option for opening up slots on the NEC. Converting the Needham Line to rapid transit (Green to Needham, Orange to West Roxbury) is also a valuable piece of that puzzle.