Will keep this as short as possible so you don’t have to read for an hour or so lol
Right now we had Classic->BC->WotlK->Cata->MoP->WoD->Legion->BFA->Shadowlands…
Classic re launched a couple years ago and was a hit for a month or so, some people still enjoy it but some came back to retail however there is no lie in Classic taking playerbase away from retail. Next up BC Classic launches same thing happens so now more players scattered to Classic, BC Classic and Retail so now less play retail. Besides both Classic releases content in WoW has been dead for a bit 9.1 been around way too long and also 9.1.5 aint giving us ANYTHING at all besides rehashed content, quality of life changes and fixes… (Mage Tower and TW Legion is all we get atm and that is rehashed content not enough to make anyone sub or sub back.) Now will I do them? Yes but majority will not.
My answer to all of this? Whelp We had the Classic re release and BC re release we will also get Mastery Classic and probably Mastery BC as well so for those that want another chance will get it that is fine. Current WOW TIME LINE IS TECHNICALLY BURNING CRUSADE if you follow me.
Shadowlands is the last expansion, if they do 1 more great that is fine but past few expansions since WoD have been nothing amazing, MoP might have been the last decent expansion and Cata is what turned things upside down 
So what happens next? Whelp our character is still fighting the Lich King at the moment, we were knocked out and the Lich King never lost, however a powerful foe came to help run the Lich King away to where he will return again. What does this mean? WoW/Blizzard gets a free pass on all the bad expansions post WotlK to start anew, we wake up from a dream world (everything that has happened) and they get to start back from post WotlK to where THIS TIME WE DIDN’T BEAT THE LICH KING but they get a fresh start of creating better expansions and new content going back to where things were a bit better system wise.
So how does the timeline work?
Classic->BCClassic->WotlKClassic->(We now don’t defeat the Lich King he flees and starting from here is all new content that should’ve happened)
Current timeline…
Classic->BC->WotlK->(The Following Expansions are us knocked out from the Lich king in a dream scenario where a cinematic at the end of WotlK Classic will play important parts as we awake)Cata->MoP->WoD->Legion->BFA->Shadowlands
