Disagree there mate.
1.4 million shirts sold per year is (was) not the upper limit for United's merchandising potential. That figure is from a 2012 Rohlmann report in Bild and IMO our numbers had stagnated in recent years (2009-2012) because despite consistent success under Fergie, we didn't have a David Beckham/ Cristiano Ronaldo type of uber marketable player. People were buying Rooney shirts, but sooner or later there's bound to be a fatigue related stagnation in numbers, especially when a lot of our acquisitions in that period weren't 'glamorous' or desirable enough.
However, with the signing of Robin Van Persie (who displaced Rooney as the #1 shirt seller) and Shinji Kagawa, our numbers jumped from 1.4 million in 2012 to ~2 million in 2014, despite a historically bad season under Moyes (official numbers as in the club 2014 prospectus - Ref. pg 21).
http://media.corporate-ir.net/media_files/irol/13/133303/140729_roadshow_lr.pdf
This jump of 0.6 million in 2 years is the kind of instant impact new marquee players can make - Van Persie as one of the best strikers in Europe, and Kagawa as a cultural/ sporting icon in Japan. In just 1 year of the two transfers, United went from having 1 player in the Top 5 Premier League jersey sales (Rooney) to 3 in the Top 3 (Van Persie, Rooney, Kagawa).