Adaptive helmet is a great idea

For two reasons

MR is an anti burst stat and stacking it does little in terms of magic DPS. Adaptive helmet reduces magic DPS without affecting (most) burst mages. In addition I believe that the 5 second buff does not refresh until it ends (and if not then it should), so you are not permanently doing less damage.

In addition it brings them in line with AD counterparts, the right click hyper carries who have two stats that counter them; armor and attack speed reduction. Now that a proper counter exists to magic DPS they can be a little less shit to balance.

This is a very smart concept. My only qualm is that it should not consider one cast of a multi hit spell like Viktor's upgraded death ray for the damage reduction, only any spell casts afterwards.