For important problems that can only be solved with expensive, multiyear projects, it's fine to pursue those large scale projects.
But this is a case where there's an opportunity to solve the problem where tolling is nearly immediate, approximately free, and probably just about as good as the new harbor tunnel would be, so we can take the money that might be spent on the harbor tunnel and spend it on something else that would be more useful (perhaps the Blue Line extension to Charles/MGH, for example).
It's reasonable to want better connections between the bus tunnel and I-90, but I think that can be done without a new cross-harbor tunnel.
In particular, once we know that we have working battery-only buses for the SL1 route (or at least hybrids that can keep the diesel engine off in the bus tunnel), we should look at building:
- a new ramp which would allow SL1 to take the left fork at the Congress/B vs I-93 split, as if going toward I-93, but which immediately after that fork would have a new bus-only right exit that would immediately descend, passing underneath the Congress/B intersection and into the bus tunnel.
- a new bridge from the Congress/B or Congress/E Service intersection toward Massport Haul Road to lead to the entrance ramp toward the Ted Williams Tunnel, along with a new portal for buses to exit from the bus tunnel to reach this new bridge
(I think something vaguely along these lines may have been in some official plan at some point, but I'm not clear on exactly what was being proposed there.)