Failed how exactly? and what does classic have to do with retail?
They are two separate games... and as we can see classic is nothing but a sideproject for blizzard since they are barely even supporting it.
Even then... failed? its been way more popular than retail has been for years. A new expansion always dies in a month or two but classic managed to chug along several months before most people quit, and even it was accelerated by content we already know how to do so there was nothing truly new there.
If they could somehow combine the two... hmm... so people dont have to launch a different client entirely.
Its never been tried before... timegating is barely an attempt to make old dungeons work, and that was abandoned real quick... now its kept as a timegating mechanic to squeeze subscription time for no reason.
If only old content could be made relevant again... hmm... i wonder how... maybe a daily badge system with randomized daily objectives to take you into older content like guild wars 2 or ffxiv. naah it cant possibly work can it? those other mmos are dead! right? ... right?!
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It died because they asked us to pay to play on them... clearly not the brightest idea to split the community.
Literally everyone was asking them to let us play on both for free rather than splitting it with a paywall. Ontop of a subscription.