Also, what do you mean by "viable"? Even if an apple is too tart for fresh eating, it will often make a great pie, sauce, or cider apple.
The apple varieties we're all familiar with are selected for transport and keeping. A commercial selection. Not flavour or sugar.
I beg to differ. There are hundreds of feral apple trees around here — old farming community — and I have yet to find one that is inedible. Most are delicious.