I agree that splitting PI calls into severe PI (spot foul) and moderate PI (x number of yards) is probably the right thing.
But I think the NFL doesn't do this unless the rules can be setup to prevent judgement calls. The NFL is right to avoid making calls subjective- if a rule is objective it greatly reduces postgame arguments. And those arguments about calls always make the refs and the league look bad. The NFL would prefer a stupid rule that no one argues about to a fair subjective rule. (One exception: the Tuck Rule).
So how could you make an objective PI rule? I really don't see any way to do it. And even the current rule is not objective enough, which is a big reason we have to wait for 10 s after any big play to see if it's coming back.
What about: no contact allowed beyond 10 yards of the LOS, period. Down is always replayed. If ref judges the defender caused the contact, 5 yard penalty, replay down. If offensive player caused the contact, no yard change (or 5yd?), replay down. This gives an objective standard and no one gains a huge advantage by contact.
If in the last 2 min of each half, clock is rewound to the start of the PI play. That prevents PI from changing the clock situation.
Intentional/egregious contact is a 15 yard penalty. That prevents either player from intentionally hitting the other to prevent a long catch or interception.
One problem is many plays may have semi-intentional contact, but the 5yards should reduce that. Ref judgement is reduced to who initiated contact, which should be pretty clear, and what's unsportsmanlike, which they already call.