Well it's just all about the bottleneck of the design.
A state-of-art independent rear suspension will be better than a state-of-art torsion beam, but we are talking about family cars here so these suspensions won't be designed anywhere close to their bottleneck. Doesn't necessarily mean a torsion beam is automatically crap, and a car is more than just rear suspension. It's about the overall package.
I sat in a last gen Mazda3 before and the rear leg room is pretty bad in it. It feels a bit tighter than the Civic I sat in. If they improved on that aspect and have to go torsion beam, so be it.