Xypherous - "We'd like to shift towards a world where Aegis is more optional overall."

This is an interesting decision by Riot, and Xypherous, it's a noble goal. So you reduce the MR aura of Aegis of the Legion by 5, but increase the MR of Spirit Visage and Banshee's Veil by 5. That way, the end game MR of the champions who build it is the same, but the MR of the champions who don't is lower.

Here's the issue: Not every mid laner can afford to build Abyssal Scepter, and very rarely do marksmen actually build Mercurial Scimitar. The support is able to make up for this by building Aegis of the Legion. In a perfect world where everyone communicated and had a good time, strategizing together and checking with each other in chat before buying items, Aegis might not be so necessary. But that's not how things work.

Just because you have the option to build MR items as a mage or a marskmen (you really don't have the option as an AD assassin, and if the Abyssal changes go through you won't have it as an AP assassin either) doesn't mean you will. And that's the whole reason the support role exists. The support is basically someone who says "Hey, my teammates are idiots who can't itemize properly, don't buy wards, and get caught out. A lot. So I'm going to dedicate my build and my time to making sure that these guys don't drag me down to Bronze III."

Runic Bulwark was an excellent example of this. While Runic Bulwark existed, mid laners who didn't build Zhonya's were actually able to handle fights against AD assassins, and marksmen were actually able to utilize their lifesteal in extended fights.

But sure, Runic was a little over the top for a single item. It was too slot efficient and had lots of invisible power.

But the support role is all about invisible power. Shields, heals, lanterns, there are times when it is very obvious that a play you've made has saved someone's life, but more often than not (a Soraka heal in lane just before a gank, a Janna shield that prevented JUST enough damage from that minion wave to keep you alive during a tower dive) the power is unappreciated by anyone other than the support. And that's fine, because support players know what they're doing is relevant. They don't need a big flashy sign that says "Good save!" every time they do something.

Removing auras one by one is Riot's method of dealing with invisible power, but we are running out of items that allow players to build utility. Some supports don't want to build Mikael's, some don't want to build Talisman, and some don't want to buy Sightstone (they'll still upgrade their trinket, but you nerfed everyone's bases so much that they're better off buying AP to make sure their skills actually do something).

TL:DR - Aegis of the Legion will never be optional. Marksmen and mid laners don't build MR, and they need it. Please don't nerf the only MR my ADC will ever be getting.