In most Central Business Districts, it is not uncommon to have a bus route on every arterial (
Chicago,
Houston,
NYC) including bus routes that run directly above full subways below). In fact, I'm hard pressed to think of a place where a subway below means "no bus" on the surface. (It is also insane that we haven't taken a lane from over-wide Boylston to run a Kenmore-South Station bus). It usually goes the other way: A subway below most often works better with or implies the need for *more* buses on the surface as demand for transit more-fully displaces SOVs.
So anyone familiar with either real CBDs or who'd ever played SIM City should have known that the SL Tunnel was not going to be able to work in isolation, to serve a "built out" Seaport (that now looms), particularly given they never built "SL Phase III" to connect the Seaport to the Orange and Green lines (and SL Phase I), but also given that the bus tunnel is really bad at serving radial commutes from the North and West (it is only good at things in the Red, Amtrak, or Worcester directions)
Better that they stake out Bus/HOV lanes on Summer and Congress now while there are fewer drivers to complain than later when, after cars have fully clogged the Seaport at all hours, it looks even sillier (to SOV drivers) to take a lane from them.
And the reality is that the BCEC is a bus destination and will always rely on courtesy/hotel shuttles for handling convention traffic. Dedicated bus lanes are justifiable on at least Summer, probably Congress, and even above the SL.