Fixing the disastrous effect that polyurethane has on floor boards unfortunately needs the use of mechanical sanding. The problem lies in that tongue and groove floorboards were never designed to be sanded, just think about when they were first put down in buildings and think when the electric sander was even invented.
Photos below will show boards that have been sanded in the past in comparison to very old boards that haven't and also the result, being the break off, of the top section of the groove.
If you finish boards in wax or oil, sanding is never needed again. The reconditioning can be done by the process of polishing or burnishing.
I will post the end result of theses, 'past it boards'.