Even with the Sox losing two home games, that's less than 4% of their home games. The earlier start date (pushing more games into cold crappy weather especially in the Northeast and Midwest) coupled with the greater number of getaway-day games (meaning more weekday day games) probably hurt the nearby businesses and tipped employees more, and to some extent the hourly employees who might not be able to ditch their day jobs for a day game shift.
That said, if the Sox did a $5 general admission viewing party and showed the London games on their scoreboard, they could create a pretty fun party atmosphere. It wouldn't draw the same level of interest as a home game but could still give the fans a little fun, keep a skeleton crew working and give the neighboring businesses a little bump.
In a way it's understandable that a club like the Padres might consider playing some of their home fixtures in another country, since they don't draw well at home...but a series like Yankees-Bosox?? A series that is probably sold out before Opening Day? Doesn't make much sense...may as well take regular-season fixtures of Chelsea-Arsenal or Liverpool-Everton or the like and play them at the Dallas Cowboys' stadium.
Manfred said they wanted the first series in London to be a big splash, pomp and circumstance, lots of fanfare. They said there's a waiting list of teams wanting to take part in the 2020 games. I'd expect to see another east coast matchup or maybe a east and a central team, with the west coast teams getting the 2020 Asia game (I believe that's going to be Taiwan), and I think Dodgers-Padres will become an annual Mexico thing given the popularity of the Dodgers in Mexico and the proximity of the Padres.