Celebi leaked

I doubt mew is going anywhere. By the way special research is set up, when celibi us released new players can either pick which mythic to study first or move down the line from top to bottom

''Catch 26 unique Unowns''

I think they will let you catch Celebi only after you catch Mew. Or they will allow both to be completed at the same time, which means that you can overlap the objectives.

Niantic have already said that no matter what happens in the future, all players (current and new) will all be able to catch Mew.

...the task is permanent for all.