The fact that the Sennheiser division has been sold recently should tell you something about Sennheiser needing to cut costs.
yes, they have been losing money for a while.
for me the explanation is not related to HD650 - more likely it has to do with Sennheiser not having successful mass-market models (think Beats alternative).
As has been mentioned. While the MSRP has remained the same they do sell via other outlets for lower prices. This is only possible when production is made cheaper.
since they made their investment back, the profit margin became larger than it was originally. later costs have been rising so the profit margin must have been dropping down slowly.
boxes have been simplified, production moved to Romania?, finish was changed also probably to make production cheaper.
the materials must have been cheap to begin with.
most importantly, the models from the same time period/price range - DT880 (still being made in Germany!) and AKG K701 are much cheaper today.
so in my opinion at least Sennheiser could move the production of old ear pads to some new place without switching to a whole different model with clear plastics. sure the new ear pads
could be just as good, but this is a classic headphone, so why change it.