Standards are always I proved upon through out all industries. Improving and making a new standard with more control, features and better sound quality. Youre talking about a standard from 30+ years ago. The Apple II was the computer standard back then.
While the DX7 was an instant classic in its day it was rendered obsolete by the late 90s digital synth boom. And whatever value it had outside of a museum was completly desyroyed by vst softsynths.
Ignoring the fact that you can actually run a picture perfect DX7 emulator on your computer, for free, with integrated midi control and an improved UI, the Native Instruments FM8 for does everything the DX7 could do x1000. Not to mention hundreds other FM soft synths and synth with FM features, many of which are obsolete themselves these days and can be had for near nothing.
besides collecting, or making a movie about the 80s, i really cant think of any logical reason a person should spend actual money on a hardware DX synth, or any digital synth with such a poor UI.