Some varieties of apple are fine on cordons, ones that spur freely with compact growth (Sunset is the best in this respect). Most apples are too vigorous and the pruning required to keep a trained form manageable just encourages yet more top growth. With the exception of just a few, most apples fare best with only light pruning to shape.
The more vigorous of my M26 cordons make shoots growth in excess of 3 feet which is just a bit too thick to prune with secateurs; using a lopper results in ugly wounds, and personally I dislike having to resort to a pruning saw. It's a sign that something isn't working.
I will give them one last chance and ringbark the worst offenders next Spring, but I will also graft up some M27 versions of these trees and either make a new cordon, or grow them as bushes now that I have some more space (or both). If ring-barking doesn't work, I think I'll take them out and plant pears in their place.