I'm not an expert either but I feel like there should be a way to properly stress test these systems beforehand so issues like this can be limited.
The issue is probably less this and more the practicality of it. I mean, just speaking frankly, patch-day issues have been present in practically every patch of every MMO for the past 20 years. It shouldn't be a surprise when you dogpile something that a problem arises. This is, frankly, normal for MMO's.

As for practicality, things iron themselves out on their own rather quickly as the "new toy" feeling wears off and the mass rush ends. Look at the login queues that happen in games (most notably the EW release itself). After a few weeks, though, things were back to normal. It's cost-disastrous to purchase enough server capacity to handle that briefest of spikes. Someone made a great analogy of buying a ship that can hold 300,000 tons of cargo. You get a ship that cost a ton of money, and 98% of its existence is only being used to hold 3,000 tons of cargo. That's an unjustifiable waste of money. You buy the ship that carries maybe 4,000 tons of cargo to handle mini-spikes, and when the rare big one comes through, people just need to realize that's life and they might have to wait a bit.

(Do people demand that airlines buy more planes or hotels add more floors when tickets and rooms sell out during prime travel/vacation times? No, they recognize that's a simple reality.)