Maple Magician Shields will be more valuable in the future?

Probably same thing that happens with Potential lock. Nothing.

You can potential lock a rare line and cube it to Unique. You'll still have that rare line. Still 3% mainstat on a 1st line unique.

So people who had their already cubed items now have(in thieves case) the advantage of a 3rd tier top/bottom over an overall(there are no 100+ tops/bottoms for thieves).
Not to mention how many other items would be affected by this change.
Kenta's goggles, VSS, scar/targa helms, strong machine gloves, most capes, most belts, most earrings, stirgemans pants, sports day equips and god knows what else all changed to second tier?
I really don't think making a ton of people lose 50-100% main stat that they cubed is a good way to make them buy more cubes.