Rian's idea for VIII was not JJ's plan

Can we stop with this nonsense, please? From before TFA's release

"The script for VIII is written. I'm sure rewrites will be endless, as they always are. But what Larry and I did was set up certain key relationships, certain key questions, conflicts. And we knew where certain things were going. We had meetings with Rian and Ram Bergman, the producer of VIII. They were watching dailies when we were shooting our movie. We wanted them to be part of the process, to make the transition to their film as seamless as possible. I showed Rian an early cut of the movie, because I knew he was doing his rewrite and prepping. And as executive producer of VIII, I need that movie to be really good. Withholding serves no one and certainly not the fans. So we’ve been as transparent as possible." - J.J. Abrams

As per The Art of the Last Jedi, Rian met with Lawrence Kasdan before The Force Awakens began filming. The basic story was locked down in August of 2014, almost a year and a half before The Force Awakens would be released. And let's not forget that J.J. was an executive producer on TLJ

I think it's pretty clear that J.J. and Rian both knew what the other was doing. Hell, J.J. even changed the ending of The Force Awakens for Rian, having R2 be on Ahch-To instead of BB-8.