No, Palpatine did not need to be brought back. Returning the Big Bad just to knock him down again is the easy answer to the difficult question of how to bring peace to a galaxy fractured by war. It implies that the only thing truly wrong with the attempts at peace was one dude who was always behind everything and if he were to die everything would be all unicorns and rainbows.
In the end, it isn’t individual people who are a threat to peace, not really. It is much more the systems that subvert democracy and the twisted incentives prioritizing armed conflict over negotiation. TLJ was on the right track with this necessary argument, despite how poorly it was implemented. Palpatine’s return, along with his magic fleet, throws this necessary perspective in the garbage.
I don’t want to prolong a debate at its end but after reading what everyone had to say for 5 pages this is the comment that resonates with me most. I really hoped the ST would at least try to tackle this concept. Lucas himself was intrigued by the idea and I think it really expands the lore from all sides where each generation faces a unique environment, giving a fairly full and rounded summary of the stages of war and peace. Though much like the reality it isn’t so easy to figure out vs destroying a boogeyman, which isn’t an invitation to have no big bads or anything, just not zombie hitler.