While I want to see a more vibrant Market East area, I think the arena proposal was insufficient to make that happen, and it had too much downside, including the impacts on Chinatown and Jefferson Station. There would be too many days with no events for the area to be hopping much of the time.
It seems that the arenas in Washington and Brooklyn have not fulfilled that promise. The construction of the Washington arena resulted in most of the Asian-American Chinatown residents being displaced due to gentrification, and the disappearance of most of the Asian restaurants.
I also found it wasteful to have separate arenas for the Sixers and Flyers. A single area has plenty of dates for two teams, and therefore fewer empty dates. While the sports complex area has not realized its potential, I think it can if all four teams are there. I think it's a wonderful and unique thing to have the sports complex all in one place.
I do like downtowns in big cities. If I ever went to Bruins or Celtics games, the TD Garden's downtown location would be very convenient for me, and that area is pretty vibrant (and better than it was years ago). But the original Boston Garden had been there since 1929, and when the new one was built, it didn't change the area all that much.