I figured you'd have understood it differently than from what it works.
You don't get assigned like you described, there's no way to be stickied to a layer. The layer you receive is one that get's determined upon logging in to play, aka a play session (as the devs stated themselves in the interviews about layering). As in, each time you log in, the system will decide which layer you will be on for that play session, until you log off, ending the session. That's why people will attempt to relog if they are unhappy with their layer, because chances are they get assigned to a new one when they log in as population constantly fluctuates.
The only way with layering to be somewhat stickied to one particular layer in a play session, is by having the first person in your guild log in to a particular layer. Because guilds will receive priority to stay in their layer (for obvious reasons), the system will attempt to make sure each guildmember gets put into the same layer as the first guildmember logged into, to make sure they can play together.
The next one in the prios seem to be groups. The person who started a group on their layer, and all the people they invite into their groups/raids, will get prio to stay in the layer the group originated from. Again, this is to make sure they can play together. If someone leaves the group, the system will redetermine where to put the solo player, which could either be the same layer as before if it's not too full, or a new layer, depending on population at the time.
The players who have least prio out of all in who gets to stay on their layer, are solo players. Those are free to shuffle around according to demand, and will be, as shown in the video here:
https://www.youtube.com/watch?v=8lH_1kUAA0g
This is what getting layered will look like in Classic. It's working as intended. And it sucks to experience, a lot. Feels very out of place, naturally, cause it never was in the older WoW's in the first place.
If guilds were to be assigned for a particular layer indefinitely, aka literally stickied to a layer making it static, not dynamic, then we'd have the situation where say we started up with 4 layers, and a guild was formed on layer 4. The overall population dips, and the game only needs 2 layers now. Someone from the Layer 4 guild logs in, and poof, there's good old Layer 4 again, and he'd be all by himself or his guild. That can't happen. That's why instead of being stickied to a Layer, they'll only have prio over groups, and solo players. That's it. It can't work otherwise.
As for server mortality, layering (aside from it being extremely harmful to the game, and completely inauthentic) only would adress that issue for the time it stays in the game, and the point where it gets removed. From that stage on, with no layering present anymore, we're back to square 1, where people once again can freely move around servers, and they have always done that in big ways or smaller ways, which ended up in having dead servers sooner or later anyway. It's completely unavoidable. People are people, and they do whatever the fuck they want. In my case, they decided to move off my once famous realm, Stormscale EU Alliance, once the MVPs on the realm left for other realms. The server died very quickly, just from that little fluctuation of specific players wanting to move somewhere else.
Layering mostly benefits Blizzards business side, and screws over their longterm core audience that they initially tried to win over and make sure they come back, and stay in Classic for its longterm success. They sure sell it well to you folks though, confusing you in how it works, promising you it's nothing like sharding, and your favorite influencers (after having met Blizzard recently in person) suddenly change their whole mindset on sharding and layerings effects, selling it to you like a piece of warm bread.
If they go through with it, we're in for a wild ride, there's no doubt. I definitely don't wanna be their PR staff when shit goes down, when people find out the game they waited and fought for for years is not like they remember, and changed in ways that echo retail very loud and clear (as its a modern solution originally used in different versions for retail).
They have more resources, preperation and tech advancements than they ever had back then, they can flip up servers which can hold multiple realms worth of populations by themselves, they are able to see name reserves, their tech is stable... All of this were things they lacked back then, and have available to them now.
Yet people act as if they wouldn't be able to handle it by letting Classic start up with normal, closed realms just as they have before, and are literally required to if they wanna have an authentic beginning for Classic, which they assured us they will provide. They have 1 chance to get it right. If they don't, they'll lose trust from their playerbase for good this time, and no matter what you guys think layering will solve oh so amazingly for you, you're gonna feel the effect of Blizzard's fuck up just as well.