The superior root system comes from the original plant, which should preferably be a seedling (which will have a taproot) or a rooted cutting of a variety that roots easily and sturdily.
So what you are saying is you will cut the graft off just below the graft site so you are in fact rooting the root stock. That is then no longer a grafted plant but a rooted cutting.
It can be done, but you will never achieve the superior root system that you get from a seedling, and seems tedious and risky. Why not plan to create more seedlings thereby increasing your root stock supply?
