It’s not about population stats.
Blizzard won’t be able to plot population metrics, ie amount of needed servers, faction balance, etc, based on “copies.”
A copy doesn’t make you choose, therefore there isn’t a sacrifice present from a choice…which can help ensure that the decision is not a capricious one.
How would you expect Blizzard to determine how many servers they need?
How many will be PvE or PvP?
How many are serious of sticking to Classic only, instead of creating insurance?
How many will want Classic only, but become bored later?
and so on and so forth.
Blizzard will do what is cost effective, will retain the most customers, with the lest amount of effort, that additionally removes as much of the guessing game as they can.
Classic ==> TBC.
Option for pure classic that will be a player choice…where you will be responsible for your choice and not them.