Honestly, they can increase server capacity, we're in the age of technology you can do that. Problem is, they need to keep it locked once they do that if not enough people transfer, you can keep increasing but people will just keep coming because it's the most populated and there's always plenty of people around ( It being the primary unofficial RP server doesn't help at all )
It's really a 50/50 fault, SE can definitely upgrade their tech, listening to their interviews you can tell they're behind, and they needed to lock it down much sooner, but people also need to understand fixes like this aren't instantaneous and crying their eyes out the first couple days won't help anything. Lock the server to anyone new, you'll force people to choose another server and for the roleplayers force them to make a secondary RP server, WoW has plenty of thriving ones, FF14 can too
