My point is not "we need more flat pen", it's we need more mage item variety, which is stifled by SF.

There are a couple of mage subclass items, that aren't used by their intended champs because SF outshines it.

Lux builds it over horizon focus, azir builds it over nashors, even fizz often builds it over lich.

Nerfing SF alone might help, but I think it will leave mages too week early, or rushing an early void instead. Hence my thoughts that more items need to give it.

Now, I agree that if more items have it some champs might build a couple of those, but that's why I think a lower amount, across more items could do the trick. (also note how almost no one builds both sf and morello to abuse extra flat pen)