The game itself was only designed to sustain about 3k people, so without layering there would either be even longer queues or a lot more servers, which means a lot more dead server in the months ahead. Layering is purely a buffer.
There wouldn’t be any ques if they limited account creations per server to maximum capacity, they would need more server but wouldn’t have to use layering, which isn’t preventing ques or overcrowding.
Then you end up with more servers then are sustainable down the line and since blizzard hates merging you just end up with a bunch of dead servers, better to layer until the population dies down and see if new servers are needed after it levels out.
It’s the best solution to a horrible situation. Too many servers = dead servers.
Too few servers = long queues
Layering allows them to watch then decide the perfect middle ground for number of servers
There was a better option, that also maintained the integrity of the world, which was to make servers ready to be merged like Herod1, Herod2, Herod3, Herod4, etc.