Into darkness is a masterpiece compared to ROS. Screen writing 101. its better to still have some explainable story Khan (some story) than no story Palatine (no story)
Khan ironically turned out to be the most layered character in the film after Spock. he had a back story that you could feel sorry for him, he had a mission and he actually did damage to entire enterprise and exposed star fleet for the fraud they could be. why is palatine back apart from the reason snoke was killed in TLJ?
. From a screen writing point, ROS will be better if they brought back Snoke.
Abrams did take time to establish Khan.
How he did it is up to everyone to decide if they like it or not but at least he did it. The criticism of Khan in into darkness was that it was too soon to do Khan as a rivalry to the classic Wrath of Khan because the young cast of the jj films had not earned it. Palatine's presences in ROS makes no sense and it destroys 6 worth of stories that started in Episode 1 and ended it episode 6.
Additionally Khan has reasons to exist into darkness as it is an alternate reality. Palaptine should be dead not just because he did die in ROTJ but because it was[B] Anakin's redemption sacrifice story. So ROS Palpatine's is objectivity a lot worse. if I was to ask any of the best screen writers in Hollywood on this issue, they will have the same opinons.