Look it wouldn't make much sense if current gear is obsolete just because of 1 expansion.
That woudl be huge disturbance in the whole progress so either we could upgrade the gear during leveling 50 -55 or..the data are incorrect.
All that "datamining" sometimes sounds/looks like few guys had too many Heinekens... and just invented something
Well so far BW has copied WoW in many ways (not all, but many).
And WoW does let previous raids become obsolete...