The problem with that is % health damage is there to help guarantee that the champion is outputting some reliable damage and not becoming obsolete at a certain point in the game.
I'd like for resistances to mean a bit more personally. But they'd need to change the scaling of it so that it starts slightly weaker than now, but doesn't diminish quite as much, for example:
Say 30 armor currently is 24%, and 200 armor is a 60% damage reduction. (random numbers)
Change it to (again, random numbers)
30 armor equalling 22%, and 200 armor equalling 65% damage reduction.
The point being that champions that do stack armor wouldn't feel as obsolete once there's an LW variant out, whilst making sure that those buying one armor item wouldn't now be tankier than before, so that carries don't run rampant building tankier (lookin at you,
)
Or Riot could just lower damage across the board slightly and make items not as effective for damage and tankiness and utility at the same time, So that people might have to actually commit to one or the other.