1. Drivers might be in less of a hurry. MBTA busses frequently run red lights and/or sit in the middle of intersections creating gridlock. [Source: Pick any busy downtown intersection during rush hour and wait <15 minutes. Watch.]
2. In my (admittedly limited) observation, Boston bus riders tend to fall into two groups.
(a) Relatively affluent riders connecting to the T (the subway or trolley). The MBTA will still get some money.
(b) Relatively poor riders commuting solely by bus to one of the multiple jobs they're holding down per day. Consider this a kind of reverse sales tax.
The best solution is to have cashless buses. In London, you can only use Oyster card or visa/MasterCard/Amex contact less payment cards to ride. It seems to have smoothed things considerably.
I wouldn't go that far either -- but why not decouple the functions of driver and ticket checker. This is already done in some systems and seems to work well. The people pay for their ticket at the station instead of the bus and stamp it themselves when then board (with a time stamp). (or avoid the process by buying monthly pass). Then people can board directly through the back doors as well because the driver doesn't need to check. Periodically, a ticket-checker or police-type of person comes through and gives fines (lets say $50) to those who don't have a ticket.