let's make the economic argument.
The fact that scalpers can get markups of 5-6 times face is pretty good evidence that demand for the Red Sox is very high. If the customer experience is not universally appreciated, then actual demand might even be higher than that.
Now, Fenway Park is one of the smallest ballparks left in big league ball. It wasn't exactly huge when it was built and most stadiums these days are built with a higher capacity.
That would fly if Boston was a small market. A small market like Tampa would struggle to fill a 30,000 seat stadium and would probably be well advised to build a smaller park than the Trop if they ever do build a new one. For Boston though, the underservice of the market by a stadium like Fenway is just unprecedented in modern baseball. This is a market that could fill a 70,000 seat stadium most nights, if such a thing could be done and sight lines, etc. preserved. It's the only baseball team in a baseball town, and besides the city itself, which could fill the stadium nightly on its own, all of the greater New England area is Red Sox country as well.
The fact is, it's just poor asset management not to build a 50,000 seat stadium with better sight lines and a few more mod cons and see if you can compete directly on an even financial footing with the Yankees (who split their much bigger market with the Mets) if you do so. It would result in a better team on the field, and now that Fenway's a protected landmark, it's not like it's going to be torn down if they do.
Unfortunately we already know it won't happen. I mean look at the name Henry chose for his umbrella corporation -- Fenway Sports Group is not abandoning its most iconic asset under any circumstances.
I would be amused if they did build a new park and move the Sox out to the burbs (say to Foxboro), and then some rich yahoo came along and tries bring the National League back to Boston at the Fens. It's not a thing I think would happen but it would be interesting to see who stayed a Red Sox fan and who would go to Fenway. Also interleague games in that scenario would be fascinating in a domestic-civil-war kind of way.