@Dottie: I am going to expand a bit on my thinking, especially so Gommié and others can take an… “expanded view”, I think one could say.
Realm population as whole can range into several hundred thousand (we know this from the time RealmPop still functioned).
Realm active population can range into several tens of thousands (see places like Silvermoon and Draenor when activity is high).
Realm population CCU cap (= number of people who are allowed in concurrently). On retail realms this number is about 6 000 (as far as I was able determine some years ago). On Classic realms this number is NOT static (and this is a major change to both Vanilla AND retail), because Blizzard invented layering.
If Blizzard elected to (and I have zero reasons to think they would) enforce a strict realm CCU cap of say, about 3 500 (, which would be at least close to original TBC) and layering entirely disabled, they would need launch literally dozens (by my moderate estimate somewhere around 100 to 150 realms each for US and EU) new realms for TBCC launch and only a few months later go on a massive connected realms spree. That does not feel like a desirable outcome (to me at least) for any party (players / Blizzard / 3rd party suppliers of various things, such as servers).
Put in other words, including several things that have already been mentioned in this thread and others, obsession with #nochanges is a massively bad idea in multiple ways (in my personal opinion).
Additional note for extra clarity (I hope at least ): Active population (even permanently) exceeding the CCU cap is not a problem by itself as queues are nothing new and it is extremely uncommon for the active player number to be very close to total existing players on a realm. However, if the active population exceeds the CCU cap (even after layering options have been exhausted) by such a massive margin that realm has (semi-)permanent (multihour) queues, it is too full and this should be avoided by any economically feasible methods available to Blizzard.
@All: I know that some people are not going to listen and even larger part of the player base will never read this text, but those who do… Please, pick your TBCC launch realms sensibly. Try to avoid the realms you know or suspect to be likely to get overcrowded. TBCC launch will bring some people back and there will be some, who elected to skip playing Classic, too.
I am not going name specific realms, because I do not have a magic ball to see into the future and into such details as how the population will split between permanent Classic era and TBCC on specific realms. All that I ask is that you consider your options and especially if you play with large groups of friends, what is best for you as a group / guild / community.