Again, out of any expansion wrath will always sustain the highest population, this has been proved time over time.
If your question is money, its not valid for two reasons
1.- Players are actively suscribed to cata , and were to wrath , just to play that expansion, no other expansion, the demand was clearly there.
Not only that but the demand for wrath is proven higher than cataclysm (or any other classic expansion to this date)
2.- Even if it was free, private realms still have a cash shop, and the cash shop in itself is very profitable to attract both players than want to play for free, or players that want to spend money similar to our current cash shop
The reason why an eternal realm specifically will only work for wrath is because of its results in terms of high population demand for that specific expansion, meaning most players playing during wrath only care about that expansion and would rather quit the game than having to play any other iteration of the game , because in their eyes, wrath is the only version of wow they ever intend to play.
Henced why so many players quit after launch of cataclysm during OG days 14 years ago , and also last year during launch of cataclysm.
If you look at the charts and compare the average population for all of classic eras and og days.
Wrath sustained the highest average population from phase 1 to phase 4
Cataclysm peaked on launch and went downhill . we currently have 1/4th or less of a population vs naxrammas during phase 1 end on wrath.
And we all know firelands is going to be peak cataclysm and dragonsoul will always be one of the worst due to having one of the worst end boss mechanics in history, like the fight is like playing gunship during wrath
This happened both in OG days and during our classic iteration, numbers are literally recreating themselves due to insanity. (Doing something again and expecting a different result )