In the grand scheme of things, my disagreement with the sentiments Reihan Salam offers here are relatively narrow, but I do want to say that I think bus rapid transit (indeed, buses in general) complements rail transit, but shouldn’t be seen as a substitute for it.
The main issue is that rail works much better as a driver of economic investment. That’s in part because it’s faster, cleaner, offers a smoother ride, and can carry more people. And in part it’s because of its permanence. A subway line is a subway line, more or less now and forever. A streetcar track can be torn up, but it’s hard and somewhat unlikely. A bus lane could become parking or a bus/HOV lane or a bus/HOV/hybrid lane or just a regular lane of traffic overnight. Consequently, if your idea is “let’s build some transit to this area and then aspire to have a ton of stuff built there” it’s much better to build rail. And in general, I think this is a good idea. Density has huge economic and ecological benefits, and the general formula of “link point A to point B with rail transit and allow more intensive development” is a great formula.
Where a bus makes more sense is where you don’t really want to do this. If you just think “hey, a lot of people live in these places and they should have better transportation options” then you want a bus line. If there’s already a bus line and you want to improve their options, then you want a better bus line. And since most American cities of any size are already running lots of bus routes, it makes a ton of sense to invest time and energy in making bus routes better, with more frequent service, faster running speeds, and a better overall consumer experience.
In principle, there are tradeoffs between these complementary ideas. In practice, they tend not to emerge. When the DC area was debating building the Silver Line out to Dulles Airport, the option of “lets invest tons of money in incremental improvements to bus service in the urban core” wasn’t on the table as an alternative. Instead the alternative was just “lets invest less money overall in mass transit by doing Dulles access with buses instead and foregoing the oppotunity to urbanize Tyson’s Corner.”