To believe there wasn’t a skeletal outline for this sequel trilogy, and perhaps further believe that all writers/directors involved weren’t at least cognizant of each others stories/plots, is a simplistic defence by people who disliked TLJ in my view.
Yes Johnson had free reign to write what he wished but, as an example of inter-play between creatives, there is evidence he asked JJ to swap BB-8 with R2 at the end of TFA. JJ was then exec producer on TLJ and, although he was hands off the actual making, people believe he wouldn’t of read the shooting script and lending an ear to Trevorrow about pre-production of 9?
There is a whole new film left to tie-up this storyline and I think, once the dust settles, we’ll be in a better place to review it. Let’s be patient.