That is your opinion, it is not that of the RFU. To be fair World Rugby's opinion may be different again, but I'm unable to find a copy of their guidelines to confirm.
Stamping / trampling has entry points from 2 to 9 weeks depending on severity. Striking with the knee has entry points from 4 weeks to 12.
PP struck a defenseless player in a place that could have caused paralysis, so I would argue that it was a top end offense. I would argue that stamping on an ankle / leg is more of a mid range offense (there are plenty more serious forms of stamping / trampling). Also, rightly or wrongly, you should bear in mind that these hearings consider the result of the offense when assessing severity - Heaslip has a broken spine, Cole wasn't injured (IIRC). You should also remember that these citings consider provocation - Cole was lying on the wrong side of a ruck slowing ball down, Heaslip did nothing to provoke PP's attack. Lastly, you should bear in mind both players' disciplinary record. I don't recall Healey having much if any previous at the time, PP on the other hand is one of, if not the worst disciplined players in the Top14.
Factoring all of the above in, I really don't see why anyone would suggest that the two incidents merit the same sanction.
I still don't see the value in comparing apples and oranges - each incident is unique and must be assessed on its individual merits within the framework I've alluded to above (again, assuming World Rugby work the same way as the RFU).