They didn't discard the creator's plot from AoE, that was the creator's plot being continued. Quintessa was one of the creators mentioned in the previous movie.

The only thing they seemingly discarded was...

1. Creators... plural... kinda implied we would be seeing more than one lone Quintesson.

2. The whole idea of Optimus ever getting any answers about how they could be created but still think they were born or why Lockdown was sent to hunt him down. I mean he found Quintessa and that's as far as it went all the unanswered questions from AoE are still unanswered because Quintessa never actually explained anything.

3. The design... cause we didn't see a lot of them other than like a hand but the little we did see sure looked a hell of a lot different than Quintessa who herself seems like a robot... though when she took on a more human form at the end of the movie it kinda gave the impression that she only looked like a robot because she was on Cybertron. It's like she just takes the form of the dominant life form of whatever planet she's on and that's not even her true form. I don't know.

Like all Micheal Bay films you leave with more unanswered questions than you started with while also not getting any answers to the questions you had before. The questions just keep piling up because Michael Bay doesn't know what the hell a plot is. Still the fact that a Quintesson even showed up in the movie is the closest the live action films have ever come to continuing a plot line from the previous film.

If they totally abandoned it like they did every other plot thread in these movies Quintessa wouldn't even exist at all.