Sad but true, most of today's electronic gizmos are made mostly of plastic. It seems that nobody expects nor even wants their precious toys to last too long, giving all of us an excuse to get something more up-to-date.
I am dismayed at a lot of Sony's current MD line-up, as most of the units look cheap and fragile. It does allow the cost to be kept lower but if these devices fail within a year or two, people will quickly give up on the MD format as being too fragile.
Gone are the days of the likes of the MZ-R50, with its all-aluminum body and sweet design; you know you have something solid in your hands with the likes of that machine or some older ones. My best advise is to enjoy your new MD units, make lots of recordings, and perhaps get a player as a compliment to your MDR.