I don't think the boxer concept is "outdated." Most piston light aircraft engines are air cooled boxers, which is a form that fits well within an aircraft cowling. Inlines and V engines really don't work well in aircraft applications.

From an engineering standpoint I don't know that I would agree that boxer engines are ideal for motorcycles. There are tradeoffs. You get good cooling characteristics from an air cooled boxer twin, but you also have side torque issues, and a more complex engine/clutch/transmission arrangement than you would have with an inline. But, BMW seems to make it work ok and so does Honda.