You need to be with your whole team to get a total shared MR of 70. Exactly the same as 
. Most of the time is just less than half of what any of those items provide. Yes, 200 gold cheaper but still, not worth it.
At least buff the active when upgraded to 
moomerator1 votesCarries need the defensive stats the most but can't afford to spend the slots on it. Yes a con of the item is that you must be with a full team for it to be useful but I think that you're overlooking the fact that an undying taric or alistar is going to be much less useful than a vayne or brand which lives for an extra few seconds. As for buffing the locket active, it may seem low but it can block a strong auto attack's worth of damage, which on 5 champions has huge potential
delonix1 votesIt's 300 gold cheaper, and it now builds out of Crystalline Bracer + Null-Magic Mantle instead of Ruby Crystal + Rejuvenation Bead + Null-Magic Mantle. I hope you were around when there were all those threads asking for Aegis to build out of Crystalline Bracer and Riot said they could do that, but Aegis would have to be weaker because of the improved build path, and I hope you said "no, do not do this, because I like Aegis just the way it is," because I should have done that and I didn't.
Banshee's Veil costs 2900g and Spirit Visage costs 2800g. Aegis only costs 1500g, so being able to have the same amount of magic resist and spread it onto your team members is pretty cool. It does suffer from the Zeke's Herald problem though, where if it's balanced around 3 people it becomes necessary for teamfights and if it's balanced around 5 people it becomes horribly gold inefficient most of the time.
I don't really understand your point about Locket of the Iron Solari. You think Aegis is too weak, but you want the active that the Locket, and not Aegis, has to be buffed?
Señor Karthus1 votesI had no idea those discussions were taking place. I was happy with Aegis before. A bit expensive to combine but totally worth it. A nerf of a potential total of 25 MR (if whole team in range) is a lot. If I mention the buff to Solari is so Aegis is not as worthless whenever you aren't with the whole team together. And same could go for banner.
Miku Lv991 votesYeah Aegis is one of the worst items in the game. But Locket is incredible, especially with the new Windspeaker's Blessing meaning it can give a 10% bigger shield and 15% resistances to your whole team. It's just too hard to try to complete the Locket since Aegis is so god-awful right now you could easily lose the game before the locket is complete in the current meta. Aegis needs to do more than it does.