I will completely grant you that St. Winston's is the most versatile choice and I'm certain it will be the most popular.
I'm just not seeing it broken. I've spent an hour+ working on builds with or without the Mark. If I have two Earcuff slots or three Earcuff slots, here's where I land.
Charm Slots - 9 no change
Ioun Stone Slots - 9 no change
Bead Slots - 7 - no change
What do I gain? A Follower slot that I was going to use Fiend to make up for loss of 2 Charm.
So now I get a second best Shirt, Leg, Hand, Head, Feet or Shin slot. None of those are better than the +5 damage I can get from the Mark. I'm maxed out on ioun stones, so Dragonkin makes no sense and Fiend would now put me over the limit on Charms. So yeah, it's a lot of versatility and as a result will likely be the first pick of most, but don't see how it's broken.
So, if your argument is it will be picked by too many and as a result should be changed, I can go along with that.
If your argument is that it's broken, I just do not see it that way.
But perception = reality. So there's that. But telling me I can beat all the other options with charms, ioun stones or rings has no merit for me. EGG is already a given. So the only choice is Arcane Earcuff or Earcuff of Orbits. If I equip both it paints me into other corners. Builds are a little more complicated.