Palpatine was brought back because they wanted to tie everything back to first six films and make it be a conclusion to the entire saga. It makes sense, if you want to end the story of the Skywalkers that you bring back the main villain who essentially started it all to be the one that you end the saga with. It helps create a connection to the original trilogy, prequel trilogy, and the sequel trilogy. All the conflicts that we see transpire on every trilogy in the saga was the byproduct of one mastermind, and only when that mastermind is vanquished for good is when the saga is truly over.
Problem is it just feels like a desperate last-minute retcon to make up for the fact Snoke was killed. It’s to my understanding that J.J had the idea to bring back ol’ Sheev since the start but frankly I’d rather they have gone in a new direction. I wouldn’t have minded if Kylo had become the main antagonist but I feel both Abrams and Kennedy wanted a more sinister presence than Kylo who naturally was more suited to a Vader-like redemption.
Another problem is that and it makes the world smaller and boils down all the problems of the Galaxy to one guy(again).
It would’ve been better, imo, if Kylo had grown into a more fearsome foe between the end of TLJ and the next episode and he was portrayed as a lost cause that was beyond salvation. Him killing Snoke would solidify his turn to the dark side and he would train in order to possess powers that surpassed even Snoke and the Emperor. Last Jedi seemed to me to be setting up Ben as the final villain of the trilogy and it seemed to be the plan until it was decided to bring back the Emperor for one last hurrah.
