Is it really an economic advantage when it's one of the only major city without a direct rail line to the airport? Honestly, in all my travels, I can get to downtown in the following cities with airports much further away just as fast as it takes me to get to downtown in Boston via the Silver Line: Seattle, Chicago, San Francisco, Toronto, Vancouver, and Hong Kong. The economic advantage of having an airport so close to downtown is overblown. The traffic that Logan Airport causes by redirecting all travelers north, south, and west of the city through downtown Boston itself limits the growth of the city.
Move the airport 12 miles west and stick a dedicated airport rail line to it and you get the same benefits without the constraints. You also free up land the size of downtown, north end, west end, and back bay combined for development by repurposing the land that Logan currently sits on.