Yes, many years ago the mantra was Microdol-X, Microdol-X,... I was taught the same mantra, and I followed that mantra for quite a while. That mantra no longer holds true because grain is not the problem it once was. Thirty or forty years ago, even a 10X enlargement from Plus-X would show obvious grain. Now, that same size enlargement from the two stop faster Tri-X shows less grain than the old version of Plus-X. If you make the comparison to T-grain films, the difference is even more dramatic. So, advances in film technology have pretty much obsoleted the need for ultra fine grain developers.
Mic-X achieved it's ultra fine grain properties by being a high solvent type developer. It contains a high percentage of sodium sulfite which ate away at the edges of the grain. The result of this action was a marginally less sharp print, and a loss of native film speed, up to a full stop off box speed, if used full strength. XTOL achieves fine grain through a different mechanism, and it does so without a loss of sharpness or film speed. Of course, one could have used Mic-X diluted to 1+3 as a one shot developer and not lost speed or sharpness, but then one would have lost its ultra fine grain characteristic.
So what did Mic-X offer that cannot be replicated by XTOL or even D-76 today? Not much. The only developing agent in Mic-X was metol, and that made it a very soft working developer. Can this action be replicated with another developer? Surely, it can simply by overexposing your film by a stop and reducing development by an appropriate amount.