Because contrary to popular belief, the majority of people who went to play on private servers weren't there because it was free but because they couldn't play the version of the game they loved.
Bfa is also part of a 2004 game btw, the game still runs on a subscription and people still pay it to this day, there isn't much difference deciding which version to play and subscribing to it.
The subscription cost dilemma is a decade old argument, to this day one can even argue that subscribing monthly to a game is a tad outdated in terms of content provided, but to be realistic here , shifting away from subscriptions to a free2play model that is heavily tied to in-game shops for the majority of it's "proper" functionality or prestige wouldn't really make it any less costly than a monthly subscription, in fact maybe even more expensive at times ( if other free2play games are any indication).
It would be better to pay monthly to maintain a pure game than to play a game that appears free but isn't and forces customers into their marketing plan, for me at least, would I have preferred if classic was free? sure. But I'm still going to pay for it.
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Ewwe
I'm not sure what reason there would be to still play on p.servers after classic releases, outside not wanting to pay the sub fee of course.
Either way the cost of maintaining classic after it's released is minimal compared to developing and releasing new content for their modern version, to be fair, pulling players back who haven't played in years is a big win for blizzard even if they don't end up staying for long, there's also a high chance that these players might discover BFA and future expansions since they're both linked , which was their strategy all along and then there's the huge content droughts in expansions which classic will help fill since it's a timeless game.
I believe blizzard's goal is to have players actually play both versions to reduce subscription downtime during droughts and to stabilize revenue streams.
But all of this is somewhat meaningless at this point, you are asking the wrong questions. Asking players to decide for you whether you should play the game or not is strange, if you wish to play it then do so, expecting to get actual answers to subjective questions from the internet is like asking whether your taste buds would prefer salty food over sweets, no one can answer that but yourself.
If you're unsure just try it and if you end up disliking it then stop playing it, it's that simple, no one can tell you if it's the right choice or not because most of the posters are here to talk about themselves and what they like, not what they think is good for you. Don't follow the masses and the bandwagon opinion about what you should do with your life, decide for yourself.