This appears to be another of the half truths that have become entrenched. I have had plenty of time to try out different techniques and have discovered that very little of what has been handed down is actually hard facts.Generally when root pruning we are told to only take off one third of the root mass, looks like you've taken quite a bit more. Is this because tridents are quite hardy?
Most species can cope with losing far more roots than 1/3. I grow lots of cuttings. I can plant a stick with NO roots and most of them survive. When you go to the nursery at this time of year they will sell you bare rooted fruit trees and roses. Most of these have had well over 1/2 of the original roots removed but have almost 100% survival so why should bonsai be any different?
Cutting roots hard at times can improve the nebari and also produce a root system that will adapt to the pot far better. In actual fact I have left quite a lot of roots on this maple. Other trees that can tolerate radical root removal are: ficus, Gingko, Swamp cypress, Dawn redwood, azalea and there are lots more. I usually leave a little more root on Japanese maples and most conifers.