Given that even the PMT imagined that there would be few net new riders for LRT to Nubian, i.e. the LRT would draw from other buses or the OL, it seems like that's not a particularly efficient investment without some additional land use changes for more density and activity along Washington St.
I think people also forgot that while the SL was an MBTA Route all of the dedicated bus facilities had to be agreed to by the City. The City, its electeds, and really the residents of the then-gentrifying South End are why those bus lanes were designed as overflow parking lanes shared with buses and bikes.
That being said, Boston has seemed to have a major change of heart over the last five years.
The reserved bus lanes need to be moved from the edges of the street (where people illegally park) to the center of the road, similar to the new Columbus Ave Bus Lanes
These lanes on Columbus Avenue are proof that the City can think better and bigger with the roadway.
What does seem appropriate for planning in the short term is a levelling up of the Silver Line on Washington Street similar to what Charlie_mta says above. If i remember well, the cost was around $10mn for 0.75 miles of Columbus Ave., Or $13-14mn per mile.
Assuming costs haven't changed that dramatically, yet, the SL Washington through the South End and near Tufts is approximately 1.8 miles. That would be around $30 million for a high class center bus way.
I do fear that the worst congestion for buses is actually from Tufts to South Station or Downtown Crossing. I have a friend that lives in the South End and works just at State Street. They usually transfer at Tufts because it's more reliable to connect with the Orange Line despite the connection distance and wait times.