I agree that the removal of the elevated Green/Orange lines at North Station and removal of the Charlestown el line is pretty much universally applauded as a wise move.
However, I've seen quite a few messages on this board and others stating that the Washington Street El should have been left in place. I agree with that as well. It was located on a street suitably wide for an elevated rail structure, and it served densely populated areas that require a grade separated rapid transit line. Unfortunately the SW corridor Orange line which replaced it is very poorly located. My opinion is that the Washington Street el could have been refurbished, possibly even replaced with a more slender and modern structure similar to the ones on the Skytrain rail system in Vancouver, BC., and it would have enhanced the neighborhoods it would have served.
As for the old Central Artery, I agree it was a hideously ugly structure, as well as having too many poorly located ramps. It may have been possible to remove it and just build a landscaped surface boulevard (without a tunnel) between the Mass Pike interchange and the Sumner/Callahan tunnel entrance, if a wider Ted Williams tunnel and a freeway connection from it to an improved Route 1 had been built. This would have provided a north-south expressway through the metro area without the need for the tunneled expressway through downtown Boston.
In any case, the wide open scar along the Artery corridor that we now have in downtown Boston is a mistake. It desperately needs to be at least partially built up with low rise buildings, and perhaps even a light rail line. What's a bit upsetting to me is that after huge areas of dense historic neighborhoods in central Boston were leveled in the 50's and 60's for a "towers in the park" type of so-called urban renewal, we are now repeating exactly the same ghastly mistake with the Rose Kennedy Greenway. Instead of using this opportunity to help undo some of the widespread devastation from 1950's and '60's urban renewal and highways, instead we are creating something that has the emptiness and soullessness of Charles River Park (which wiped out the West End) and Government Center.
Instead off re-knitting and healing the scar created by the old Central Artery by building a lively, diverse, and walkable neighborhood reuniting the cleaved halves of the city, we're building an empty, wind-blown no-mans land, which will get some limited use in the summer, but I expect very little use in the winter.