The problem is in a heavy landing, everybody is looking outside and nobody will glance at the VSI, so unless the aeroplane has a flight recorder than will spew out the figures, it takes an estimation of how hard ones backside was compressed to decide if a heavy may be on the cards.
I think inertia has little to do with a heavy landing. Light or heavy, the descent of the aeroplane is arrested in a few inches, so fuselage bending moments will be unaffected. A large amount of fuel will simply stress the wing structure. The major determinant of Max take-off weight has to be structural strength of the Gear, with wing strength and lift having a lot of effect too.