Yeah private Vanilla servers are still very popular, that is why Blizzard decided to release Classic servers ( starting with patch 1.12)as naturally they want their piece of cake, as they saw that there is still huge demand for Classic Wow.
Regarding asking for money, all you have to do is to have active subscription and you have both retail and Classic WoW available.And if you dont want to pay WoW subscription with a real money, you can always pay with gold, and it is fairly easy to farm it in retail WoW.For the last 6 or 7 years i haven't spent 1 cent on WoW, all my gametime comes from ingame gold.
BTW, if someone plans to play Classic WoW and wants to reserve some cool name, you can already do it, since last night.You can only reserve 3 names though.
OK, about how much in game time do you need for the game to pay itself? I imagine this is farming items and selling them, but is this allowed in game or outside? I remember games like EVE online kinda allowed it in game but not sure about WoW.
Time needed to farm enough gold for subscription depends how many characters you have, right professions etc. The more experienced and set you are, less time is needed.For example i am passively making every month for approximately 2 monthly subs(worth 30 dollars), by simply logging in every day with my 10 characters and spending maybe 10-15 secs on each character every day, just doing their profession things. So basically negligent amount of time, but of course for that you have to set it up first.
But if you are good and skilled, you can make much more by boosting people for gold, and that is also totally legit by Blizzard.
For new players that can only do basic farming , i would say it needs maybe 1 hour each day , farming herbs, ore etc.
All this is done in game, and Blizzard added option to buy game time with gold few years ago, so everything is totally legit.And not just that, you can buy any blizzard game with the gold(Warcraft 3, Diablo 3, Overwatch, Starcraft 1 &2,WoW, Hearthstone card packs and even few partner games like COD, Destiny 2 etc).