I said "typically." Sometimes they're after the ptr but they definitely aren't MONTHS after the raid hits LIVE servers. And some of these dates are wrong. MOP announcement was in October 2011, 4.3 ptr was THE SAME WEEK. And 8.3 didn't get previewed until the January after the Shadowlands announcement: there was no 8.3 content discussed at that Blizzcon.
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It's not optimism when I say if Blizzard can release an unfinished expansion, they can definitely announce an unfinished expansion. Remember what they showed with the pre-order of shadowlands? Like 2 second of Kyrians lounging. That was it.