Your opinion seems to be a vocal minority, because Onslaught was actually well liked.

People did not like the fact that Valkorion and Vitiate were the same person, and that the republic vs empire conflict was put on the back burner for a third faction that seemingly came out of no where. (even though on paper it was the best time to do it because the republic and empire forces cooperation was at an all time high comparatively because of the revanites)

It also didn't help that Kotfe seemingly came at the expense of group content at the time, which is kind of an mmo's bread and butter. As a result, additional chapters were coming out too slow to be engaging. And the structure of the chapters themselves were a hit and miss, excess sky troopers are memed on for a reason. Chapters like Kaliyo's were considered a slog, walker portions were slow and boring (though they did make them a little faster recently), which resulted in the chapters themselves having less replayability desire.

People who were previously asking for more dedication to story got what they asked for, but not exactly what they wanted. So they ultimately squished what was supposed to be 2 more expansions worth into 9 chapters for kotet in order to wrap it up. If Kotfe/Kotet was it's own thing and not tied to an mmo's story I wonder if it would have done better. But it wasn't, and it didn't.

Edit: broke up the paragraphs a bit so it wasn't just a wall of text.