Actually, Concorde was too big: 100 seats. It worked NY/London but at ~$20k/seat they couldn't fill enough seats several times daily on other routes. Which means you can't have many airplanes, which means no economies of scale.

With 40 seats and business class prices, the Boom airplane works on many more routes, which means we can make a lot of airplanes and enjoy economies of scale.

Looking into the future, as fuel efficiency further improves, per seat costs come down, and it will make sense to make ever larger aircraft. It's a virtuous cycle that will eventually allow supersonic flight at economy prices.