Yes, the existing lenses out in the field certainly kept Nikon and Canon in their position. Anyone with a large amount of lenses would stronly prefer to upgrade to a compatible camera.

This is what makes their move to mirrorless so tricky. While adapters for the existing lenses exist and work, in the long term you would want to upgrade many of your lenses to their mirrorless counterpart. Especially, if most new lens designs are made for the mirrorless lines.