Let me clarify my comments. I agree that it has been one of the sources of the deterrent effect, but the way it has been interpreted, particularly after events in Turkey, as being an ironclad guarantee of a response isn't quite right. If you look at the actual article, you see that an attack upon one is equal to an attack upon all; however, for it to be activated, members of NATO have to agree, first, that there has been an attack. It may seem obvious in some cases, but if you go back to your League of Nations example, that was precisely the debate that was had about Manchuria. Was this aggression or not? Was this an attack?
Second, they have to agree that NATO will respond. That's a political decision.
In addition, article 5 doesn't specify the action that will occur. Each member does what it deems necessary. We know that in the case of Afghanistan, countries behaved very differently in response. It is about managing expectations around the type of action that might follow.
I don't at all advocate moving away from or lessening the importance of article 5 for NATO. I think it has been key to deterrence. My point about Georgia was just more caution. With caution has to come credibility, the credibility that you would activate that clause. At present, I don't see the same commitment to the security of those countries that existed for some of the earlier entrances to NATO after the end of the Cold War.
I just sound a note of caution about expansion. You're right that it could have a positive effect, but not if it is thrown around willy-nilly without the accompanying credibility of the commitment.