Even going by 1994 attendance, Alumni field would be undersized by 10k or so seats. Soccer's popularity has only grown since then. Even notwithstanding the problems with fitting a regulation field into it, I'm quite certain that, despite being a top flight FBS program, the press and broadcast facilities at Alumni don't hold a candle to what's in place at Gillette. For an event that the world's sportswriters will be descending for and putting out to a global audience, even if the main broadcasting hub will be out in LA? They'll probably want the superior NFL spec, as it always uses more cameras, has more broadcasters present, and has higher standards compared to any NCAA program. Even Gillette probably wouldn't be able to handle it purely using installed systems, but that'll depend on how much gets centered on the IBC in LA.Instead of using Gillette, couldn't they use Alumni Stadium at BC? Sure, it's smaller but it's right on the green line and it would be easy for fans to reach the game by public transit.
For an event like this (and the Superbowl) where most people *aren't* watching in person, the broadcast capabilities matter just as much if not more than the people in the stadium. See the 2020 Olympics, (starting in 6 hours!) That's all designed for broadcast; the majority of viewers for major sports in modern times have all been on the other end of media, and the choice of stadia or venue fundamentally doesn't impact the experience for the majority of those watching. The ability of the venue to support those broadcast activities does. For the 2012 London Olympics, the main stadium cost ~£490m. The Broadcast center cost ~£355m. That's how important that is to events like this.
Besides, if we're making this about public transit access, it's not as if game day specials to Foxboro from Boston and Providence aren't a thing. Foxboro might even have regular service in 5 years.