What? We've had multiple deaths that haven't been reversed. Yes, characters have come back, but they were temporary and involved spirits (Or it was the Legend War, where trying to wrap your head around how some characters' returned would be maddening. Probably "Super Sentai Spirit" or some mumbo jumbo. Case is, they still stayed dead after the war ended)

Mostly true. For years, each installment was within ITS OWN storyline/universe. The "VS" Movies were the only genuine instances when the rule saw its exception. Gokaiger was the game-changer by making VIRTUALLY EVERYTHING canon. Then it reverted to normal format until Ninninger decided to mesh it all together again WITHIN THE SERIES. With that said, AkaRed could appear because Gokaiger changed the game by including the old within the new. The loose continuity of Super Sentai now has to make sense of its long and occasionally contradictory history. It is no longer beyond the scope of logic to take an older character and implant them where they simply don't belong.

I never feel like any of the Sentai shows had major continuity with each other besides Sun Vulcan and Denziman. The idea all of the Sentai in one continuity since there would be too many plot holes to work with.