I doubt Lost Ark will be a total flash in the pan but it'll probably hover around the BDO-level of playerbase after the initial boom.

World of Warcraft lost 3 million players last year, and not all 3 million of them came over to XIV. While XIV attracts a lot of people for its story focus, it also repels a lot of people for that same reason. I think the current MMO climate is very different than it has been in a long time, simply because the whale carcass that is WoW has left a lot of people displaced and actually looking at a new MMO for once instead of just immediately writing it off as "Not as good as my home MMO."

It'd be like if XIV's servers suddenly shut down for good next week. While initially you might have overlooked brand new MMO #45 because XIV is your baby and nothing could take you away from it, even if it was actually pretty solid, now you go "Hell, why not? I got nothing else to do" and you sink your teeth into it.