Actually the design flaw is more about quality of the bearings and of the duff original factory settings and workshop manual instructions than any grease. I've had three sets of bearings fail (we did experiments, don't worry about that, and it cost me nothing) going by the original instructions and later suggestions from Piaggio. All but the first set were Very Well Greased.
I humbly submit greasing isn't the problem except for the first set from the factory - and even that situation may now have changed.
If those front wheels have been over any 'hammering' surface, the OEM bearing races will very quickly become compromised. If your MP3 is the one I think it might be, it did loads of rough roads, so not surprised they've pegged out.