The proportion of fragments that would have their orbits boosted, through multiple collisions, to an orbit higher than the upper atmosphere, is trivial. Nearly every angle of collision between two objects in orbit lowers their periapses. The risk of Kessler Syndrome doesn't come from objects in upper-atmosphere orbits somehow getting boosted out through collision chains, it comes from collisions between objects already in higher orbits not strongly affected by atmospheric drag (>600km).