How did Blizzard handle the TBC and Wotlk launches without layering?

No it’s not overly simplistic.

I can maybe point out why they’re doing the layering thing with Classic in phase 1 only.

Simply because they’re having a problem gauging the number of actual servers needed.

Laying gives them the option to expand or reduce the number of servers with out impacting the community as much come phase 2.

How effective that can be is yet to be seen, but I discussed it further in the following post and simply copy / pasting that over here is redundant, so here is the link to that discussion.