I really can't buy into this reason because they used to do beta without the subscription requirement. It seems like if they want to test how many people are interested, they can just open it up to everyone like they do for every WoW expansion. And how are they going to know how many servers they need for all the people without a subscription who can't reserve a name or qualify for beta?
Long-expected World of Warcraft Classic is confirmed to come in summer 2019. WOW Classic major content will be released in 6 phases, and you could learn the schedule and some information on character creation below. When is WOW Classic release date? World of Warcraft Classic is planned to release this summer on August 27, 2019. This will be a global release, and you could learn the times for each region on the...
*If you run out of Bag and Bank space on your Alt, fear not, as the Mailbox itself is a great place to store your Items. Just mail everything to your Alt and keep it in the Mailbox. Expired parcels will be automatically mailed back to your Main, so you will not lose any items and be able to Mail everything to your Alt again (Mailing items cost some additional Gold, but the amount is negligible when compared to the utility provided by almost unlimited storage space).
This is the lamest excuse to try and pin it on a cash grab. Tbh this is just you complaining to complain, there is no issue. If you wanted to play classic on launch you'd have to sub before hand anyways you're still paying month to month you don't pay by week. The only way you'd lose that money is if you quit before your sub ran out later on down the road. 
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