because blizzard admitted that removing layering will increase queues.
let me clarify some wording here.
sharding ensures a zone is not too crowded.
layering allows a single server to exceed its normal capacity.
a realm as we know it was designed to hold around 2500-3000 people.
a server though, can handle a much larger amount of people.
what layering does its creates multiple copies of the same world, such that a single server can help a realm hold more people via those copies.
so if a single layered realm would only have a cap of 3000, a realm with 2 layers would have 6000 people cap. 3 layers would have 9000 cap on it, so on and so forth.
when blizzard gets rid of layering, they are not having mega servers with 9,000 or 15,000 people online. they are going back to that cap of roughly 3,000 i mentioned before. all those extra people? where do you think they are going to go? they will go to queues.
if you want more information - here is blizzard saying they expect severe queues when you remove layering (here is looking at you, faerlina, herod, etc).
if you want information on how layering was designed to work and the removal window, here is an interview with Ion, complete with him explaining how a few weeks after launch they will be collapsing down on layering.
