Totally agree, but as you rightly mentioned, that's from pure audio design point of view. Computer case design is a totally different thing. Normal PC case design key objective is compact. And in most cases, cd drives are right next to the power transformer which generate huge vibrations and electric noise. So, most of the CAS users suggested to isolate the drives from the PC cases. And that's what I try to test it myself whether it's true or not.
And you've got a good observation that after take the drives out, I worry about it's own vibration as well, so, I cut 2 EVA plastic to take up the vibration together with the monitor as well. But as mentioned, after all these, the improvement is very minor