Your arguments however for solely AD champions also works for champions that are solely AP. While I 100% agree with you that Magic tends to be more AOE than physical and that physical is aimed at more at one person I don't think that's grounds to justify that if someone is squishy they should straight go down to AD. In an offhand example that's like saying hey lets bully the only white kids in a dominantly black school because there are very few of them. Why discriminate? AP can just as easily be applied to solely one person main offender (Akali and Leblanc but she does have Aoe).
Let's say for example that was ok, "Your squishies shouldn't survive because you bought an armor aura for them they should survive because they positioned well and received peeling". If were going to use that as a rule then by the same token, "Your squishies shouldn't survive because you bought an magic resist aura for them they should survive because they positioned well and received peeling" main offenders being (Leblanc, Katerina, Akali) should work as well right? I don't see how having armor aura would make the game boring. You buy items to suit the purpose.
The items we are discussing here are all situational. If my team is getting screwed by Ap as it was it wouldn't have the best thing for me to buy a frozen heart where only Irelia and Varus are the main AD offenders and also have magic damage build into their kits whether it be through ultimate or other skills and were the ones getting to us. Again those items you listed aside from the crucible and and face of the mountain which I also made arguments against in a previous post (frozen heart only when most of the enemy depends on AS) benefit you more than the team situationally. Yes you're right. In the middle of the fight soaking up damage for the team sunfire, thronmail and frozen heart are great but for the most part good players know well not to focus you. As you stated before good placement will make it so they will bypass you to get to those who aren't there to soak up damage which is why it's good to have aura items. In the case of the enemy team versus us who had more AD than Ap buying a sunfire, randiuins or a thorn mail for the tankiest person on their team who it would have benefited the most from buying it irelia would have been more effective against us than her buying an abyssmal scepter even though it would lower our magic resist or for that matter chogath who didn't buy that item even though that would have been a great help to his team rather than him buying the warmorgs which while it kept him alive did not keep him alive long enough for us to slaughter his team and then take him out and would have worked better on Irelia.
I like your arguments however as I stated already they can also apply to AP champions and I don't think they are justifying not having an armor aura item. It seems if anything your arguments are advocating for the recent decline in assassins which isn't a bad thing imo. Tanks are just shining right now but there time will pass eventually.