I think Ridley planned several sequels after Covenant and after the average to mediocre response, he decided to take a different approach and condense all ideas of those several planned sequels into one final movie, that is what I think the case with the sequel is. So, naturally, that would take a bit of time, especially script wise, they had layouts for several films and now they need to put all that into one single canvas. I doubt Ridley will let the story unfinished.