To me it sounds like they did have a plan, saw how it was being received and adjusted it accordingly based on audience reactions and Fisher's passing.
Trevorrow was fired *before* TLJ even came out.
His firing would've had nothing to do with audience reaction to TLJ. In fact, his firing pretty much coincided with their announcement that they were giving Rian Johnson his own trilogy. Fan reaction would've had nothing to do with why they didn't want to go with Trevorrow's story.
I was always one who believed they *had* to have a plan. Especially when they said that the ST movies would go: VII.) Han-centered, VIII.) Luke-centered, and IX.) Leia centered. But it's way too obvious at this point that there was no real plan, and not even a story outline for the ST.
When they decided to scrap all of those years of EU canon, my assumption was that they were doing that so there could be one singular and cohesive universe where the stories would be interconnected between comics, novels, shows, and movies. It made all the sense in the world!
When the comics and novels both had the Palpatine contingency following ROTJ, it seemed to me like the one cohesive vision was being built for how we got to the ST situation with the FO. The "Unknown Regions" started popping up everywhere in printed stories, and Snoke was starting to be portrayed as someone from there who had observed the Empire and had been familiar with Palps. When Sheev was laughing in the TROS trailer, I thought for sure this would be the payoff, and all that Contingency stuff would be revealed on screen. Nope! Palpatine survived all the while and *was* Snoke.
No plan; no cohesive vision between the movies and the other canon; no cohesive vision even within the three movies themselves. Just a complete and total mess. Embarrassingly inept leadership with no true vision and direction. KK and Iger blew it.