The White Council is strong enough to face most of the foes we know of.
I dunno about that... in fact, I kinda doubt it.
We have multiple voices saying that the WC is teetering on the edge.
Being strong enough to face most foes and being rotten in the core and collapsing are not mutually exclusive.
I think the WC is both strong enough to face most of the threats out there AND teetering on the edge of collapse. It comes back to the fact that there are individual members of the WC that are absurdly powerful.
If you give the WC a target and let them bring the Sr Council to bear on it... they will take that target out. There are exceptions to that, but even then... they might. That doesn't mean that the Council isn't actively collapsing. It doesn't even mean that the war with the Red Court didn't EXTEND the life of the Council. Having an external enemy may have kept the council together through a time it would have fallen apart in without that. Infighting and political maneuvering are likely to end the council within Harry's lifetime.
I can't help but draw parallels with Issac Asimov's Foundation and Empire... Spoiler warning for a 75 year old book... The Galactic empire is DEAD... it has been collapsing for hundreds of years when the story starts... its another 200 years later that the Foundation (the colony that the story follows) first encounters the empire and when the two get into a struggle the Empire wins EVERY fight... and just before the empire wipes out the Foundation it withdraws it's forces... and this is all as the mathmatics of history predicted 250 years earlier... if the emperor is weak then strong generals would attack the emperor, if the general was weak then the foundation could defeat him or he would be held in check by other generals and governors... only a strong emperor and strong general could possibly threaten the foundation, but in that case the emperor would be forced to recall, accuse, and execute the general or be deposed by him (the conquering hero). The Empire was stronger than any other faction in the galaxy... even stronger than the Foundation, but it was already dead and had been for 500 years. Once the empire had directly governed every planet in the Galaxy, now it governed less than a third and much of that only indirectly. Once the Empire had built fleets of warships stronger than anything ever seen before or since, now they couldn't even fix them when they broke, much less make new ones. Oh, the knowledge was in books, but no one had the technical understanding to read those books and put them to use. And more often than not the books themselves were hidden and guarded.
The WC is a lot like the Empire of the Foundation series... Including the Merlin kicking out Harry for being too successful out of fear. Arthur Langtry is the most powerful mortal practitioner alive, but he isn't a great leader, which is what you would expect from an organization that promotes members based on personal power/skill not ability to lead and make decisions. He seems to be driving the WC off a cliff for fear of not being the guy in charge... He is too defensive, too inactive, too afraid.