[FairlyInvolved]

I don't think debris 'going higher' isn't much of a problem. Whenever this happens the orbit is going to be more eccentric - meaning a lower periapsis, and consequentially lots of drag that will cause a rapid deorbit.

On the second point about parabolic orbits I also find that probably relatively low risk because we are only talking about a fraction of an orbit for a collision to occur so unless the debris field was massive the chance of another collision is probably still low. Remember when we are modelling orbit collisions normally we are often talking over 25+ years - 100,000 + orbits.

I think the main problem is busy orbits (e.g. sun-synchronous polar orbits at popular altitudes) where most of the debris remains roughly in the same orbit following an acute collision but has a lot of other potential collision targets. Also as satellites are disabled by a collision they lose the ability to avoid other objects already in the same crowded orbit - i.e. the fraction of objects able to take avoidance decreases increasing the chance that future collisions are from 2 incapacitated satellites, removing the possibility of avoidance.