Can we have a 100 MR item

No, the problem is AD boosts all auto attacks meaning AD champions will always have good autos whether burst or not they will have some sustain damage output and most have a form of sustain to heal. However, burst mages don't have either and having their entire combo nullified and unable to do nothing would ruin those champions and is bad for the game. Therefore they can't have a HUGE MR item like that.

Can we? Sure

Should we? Probably not

Banshee's is pretty much that equivalent. Stops a spell or combo where those abilities have CDs (and banshee's CD to match) vs autos that have no cooldown and no CD on thornmail to match

I'd rather have an AP-focused 2nd tier item for merc sash