[ghaff]

I'm not an urban planner but it's hard to separate effects.

You also have the ongoing Seaport buildout and, as you say, Boston (and Cambridge) was well-positioned in any case to take advantage of technology/science industries and many educated young professionals moving back into cities. (The Cambridge biotech/pharma buildout had very little to do with the Big Dig for example.)

On the other hand, the Big Dig did improve downtown and airport access in various ways so I'm not sure the degree to which the Seaport expansion would have been possible without fairly large-scale road work. (And, yes, the public transit infrastructure done to service the area is sub-optimal.)