DX7, being a synth, can have it's sound emulated. So I don't think it's something people would go searching for badly. You can have similar sounding machines with greater flexibility, control and polyphony nowadays - no point going after the DX7 unless they are collectors.
KX88, being a controller, is different. It was build to last and many people still uses it. If I can get my hands on one, I would! But it's hard to find KX88 nowadays. People who owns them don't give them up! And Yamaha don't make controllers like this anymore.
You may want to upgrade your synth with new engines and sounds etc, but you only need one good controller - one that gives you enough power of control, good touch and sturdy. That's KX88! I still own my Roland A70, although I wish I have the A90 with weighted keys. But it's hard to find the great feel of the unweighted keys like the A70 nowadays. That's why I don't give it up.