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.