Perhaps we should look north?

What is the best analog for the future of transit in the US? If you ask the ITDP, they point south: Mexico, Colombia, Brazil, countries which have invested heavily in bus rapid transit. It's a rather curious comparison for a couple of reasons. First, those countries have much lower standards of living than the US does, so that operating costs for high-personnel transportation systems (buses require five to ten times as many drivers as trains) have a lower cost. Another is the streetscapes: many South American cities have Haussmann-style boulevards with plenty of room for BRT systems which are wide enough to support high-capacity systems. We don't have that in the US. There is also a climate question. Countries with no freeze-thaw cycle can more easily build asphalt and concrete busways and expect them to last, while in the US, they require more preventative maintenance.

So perhaps the best place to look is not to our south, but to our north. That's right. Canada. Unlike the US, no city in Canada had a pre-war subway system. When demand outstripped street supply, Canadian cities have taken a number of different tacks towards addressing transit needs:

• Montreal's looked to Paris and its Metro system is today the third busiest metro system in North America, with more than a million passengers daily, more than all but Mexico City and New York. It's bus system carries another million and a half passengers.
• Toronto's Rapid Transit system also carries more than a million passengers per day, and is supplemented by the city's extensive surface network, which carries nearly two million more passengers, and Go Transit commuter services, which launched only in the 1960s and carries nearly 300,000 passengers, and is being upgrade to electrified service to provide faster and more frequent trains. All told, Toronto has nearly 3 million daily riders, and TTC's 73% farebox recovery ratio is the highest of any system in Canada or the US.
• Ottawa's OCTranspo carries more than half a million passengers daily, nearly all of them on buses. Many of these buses use mostly-grade-separated transitways and Ottawa is the only city in the ITDP's database (scoring bronze status). Yet Ottawa is working to convert most of it's bus rapid transit to light rail. Why? Capacity. The downtown segment, with 100 mostly 60-foot buses per hour, is oversubscribed. Even the ITDP admits it (although their circular logic is that this just points to the success of BRT, although they don't give any solution for increasing capacity). 
• Calgary and Edmonton both have light rail systems. Calgary's is very well patronized, with more than 300,000 daily passengers using a single downtown trunk line, and illustrates that surface-running light rail can far exceed bus capacities in a limited corridor (including double the capacity that the ITDP quotes for light rail). Calgary's system is now the most heavily-used light rail system in North America (yes, more than the Green Line in Boston) and both systems are expanding.
• Vancouver hosts Canada's newest rail system, Skytrain, a "light metro" system which is fully grade-separated but has slightly less capacity than a full metro. Most of Skytrain is above-grade, although several sections are in tunnels as well. Notably, it is fully automated, reducing operation costs further. The city has several very busy bus routes (particularly the 99 to UBC) which are planned to be replaced by Skytrain lines in the future.

Unfortunately (or maybe fortunately) the ITDP hasn't studied these Canadian systems, because—especially in the case of the newer systems outside of Toronto and Montreal—they don't fit in to "BRT good light rail bad" narrative. If Ottawa had built light rail like Calgary, they would not have the capacity constraints that are now causing them to build a light rail line instead. Downtown Calgary is booming for a variety of factors, but one is that its light rail line has the capacity to serve the number of commuters necessary to support the dense downtown without highways. It would probably be booming similarly had BRT been build instead, but it's likely that system would be over capacity and, like Ottawa, would need significant enhancements or a mode change. Montreal had a bus rapid transit line that they're rebuilding on a peripheral street for US$25m per mile, although costs are ballooning and it's a lengthy project, and Montreal's trunk lines are still served by underground rail.

Canadian cities have generally been successful in getting people to commute without driving, much moreso than in the US. Even in the Texas of Canada (Alberta), half the commuters to Downtown Calgary take transit to work (and Calgary does not have many large peripheral office parks; development is focused downtown). Bus rapid transit has worked in Ottawa, but congestion, especially in the downtown area, has caused the system to be replaced with light rail. While the outer portions work with buses, where many routes merge and streets get narrower (and Ottawa has a grid street system downtown with relatively wide roads) it has been overwhelmed.

So, back to Boston, a city which is more similar to Canadian cities than it is to, say, Mexico or Curitiba. Canadian cities have somewhat similar development patterns, highways (Montreal and Toronto both have highways serving downtown and terrible traffic, does that sound similar?) and standards of living. The ITDP chooses analog cities of Cleveland, Mexico City and Belo Horizonte in Brazil. These are examples of cities which have bus rapid transit. I'm just not sure how well they compare to Boston.