At this point, I think it's a matter of time for meeting the ITDP BRT standards.
If / when the Columbus Ave center-running bus lanes are extended north toward Tremont St and Ruggles Station (which the
MPO has agreed to fund), it'll almost break the 3km / 1.9 mile mark. Blue Hill Ave, which just
received the RAISE grant from USDOT, is just under 3 miles between Warren St and Mattapan Sq. The SL4/5 is about to turn 20 - I can see all these center-running projects nearby sparking some local conversation for rehab'ing Washington St (it could also make it easier to look at better biking facilities, which are also badly needed there).
As a side note, I think that there's too much weight given to the ITDP gold standard. BRT is great, but there are plenty of instances of good bus service (thinking London in particular), where there's no hand-wringing over whether it's meeting each of these standards. The focus first and foremost should be on providing frequent, reliable, safe, and accessible service that riders need.