No personal attack on you
@DZH22 but I think you're just missing the point of this stadium entirely. You're trying to dig your heels in on the American exceptionalism angle that centers/ glorifies burning fossil fuels in smokestacks or car engines and sports that are the best in the world because no one else cares. What brings people to soccer games is not just a winning team but entertainment value, atmosphere, and community. You're completely off in terms of thinking it's not a good product.
I've been a massive fan of the MLS for the past 15 years now and you can't deny that the sport is popular and growing rapidly in the United States, even though the league isn't competing on the world stage (yet) they still attract exciting young players and massive stars because of the standard of living and the marketing opportunities (arguably the best soccer player ever is in the MLS right now). Even right now with the "rankings" the Revs are in, they average 23,000 fans in a football stadium which is not ideal for soccer. The model for success for soccer in the US for the past decade is 20,000 seats close to the city, with access to public transportation and things to do in the surrounding area. So trying to point to global rankings as a measure of success is entirely irrelevant, this team will be popular here and I don't think it's correct to think of it as a small niche. And in any case, the best soccer teams in the world build stadiums 3 times as large as what is being proposed here.
Earlier I shared similar stadiums that are currently being built in NYC (11 major league teams) and Miami (6 major league teams). Let me also add to the list, cities that have recently built very successful soccer stadiums of a similar size:
Los Angeles (10 major league teams),
Minneapolis (5 major sports teams), DC (7 major league sports teams), and a smaller one
Cincinnatti (3 major league sports teams). Click the links, all of these teams draw great support. Why not Boston?
This stadium would be a hit from day one and only gets better looking towards the next generation of young and diverse Bostonians. It makes sense economically, it makes sense culturally, and it makes sense in terms of infrastructure and development of this area in the long term.