Revert Wits End But Keep MS passive.
Few obvious reasons.
Morellonomicon counters the healing passive.
Stealing mr to amplify your own damage against an overpowered mage is always better than taking flat mr and hoping they won't burst you through the sustain.
Gives champs like Kog'Maw, Varus, Corki, Udyr, Warwick a more reliable way to get their damege through. Keeping the movement speed on will help bruisers get in range of mages, which they already have a hard enough time doing.
While you're at it you can get rid of the shred stats on guinsoos so adcs don't pair the items and become fucking broken like two years ago.

for some of the above champs. It also helps them deal with hard AP counters or scaling ones like
and whoever else.