Unless you have deeper insight into the subject and can actually showcase why TBC needs additional funding in comparison to Classic, i wouldn't necessarily throw that arguement around.
Especially when your assumption goes against the word of the people that are actually working on it.
Face it, what you're saying just goes against common sense.
1. TBC is a much smaller project than Classic
2. Classic did not have any additional monetization outside of the regular sub and was still considered a huge success by Blizzard
3. The far simpler explanation would just be that Blizzard wants more money
I have no idea why this is the hill you want die on, especially you yourself already acknowledged the far more simpler explanation.
This isn't about how much effort it is to implement TBC, it is the big question why does Classic did not have any additional monetization but TBC has.
To which the most reasonable answer is: Blizzard wants more revenue, not because TBC takes more money to develop (where also the more logical monetization would be a one time purchase, not a deluxe edition or cashshop).
Because they were asked about the option of potential TBC servers, to which their respond was: "The legwork is done".
People did not ask for details on how they will implement TBC.
Why should they go into detail when that was never question?
If TBC was actually much more costly to develop then the statement "the legwork is done" is an absolutely misleading statement, if not an outright lie, and when it comes to technical stuff, i am more inclined to believe Blizzard when it makes sense.