It's been proposed a few times, it would also eliminate the need for the difficult to procure and aging dual modes that are limiting the Ts ability to run more frequent SL service. However, so far it's been shot down because of potential customer confusion of when SL1 buses will be arriving at surface level vs underground and how to direct people to know which way to go, and also how to maintain connections for Seaport riders to Logan.
SL1 should stay intact. The surface-level express would be an additional "premium" express service run by MassPort. During peak times, SL1 gets so packed at SS that no one at courthouse or WTC can get on anyway. The express would divert some of those SS riders and free up room.