Roger Federer played squash as a kid/teenager. There is no two handed backhand in squash because you have to change direction quickly.
Wawrinka, Almagro, Gasquet, Roger GOAT Federer are all playing top tennis with their single handers. How can one argue that it is inferior? I would love to see an empirical/longitudinal study on single handed backhands. Surely someone is collecting this data at the juniors, college and pro levels.
I don't see the "easy to learn" as a valid point. Any tennis player who is worried about which shot is the best is going to spend time honing his/her skills.
More important question: Why do Americans, at the pro level, have such poor backhands regardless of how many hands they use?
Disclaimer: I'm NOT a coach...
Real tennis: https://www.youtube.com/watch?v=yDqnkLJ9BtM