Does cancer that has been in remission always come back?

It depends. Remission is a somewhat vague term. In theory, if a patient dies of something unrelated to the cancer at some point in the future, the cancer never recurred. So it is a function of life expectancy, and the aggressiveness or type of cancer.
No. We are often afraid to say "cure" and "remission" seems safer when describing the absence of signs & symptoms of cancer on eval, tests, etc during followup for a cancer patient. Stay positive & address mind & body well-being as we eval over time confirming no evidence of cancer. Confidence in saying "cure" is very much related to exact type of cancer, stage, treatments, etc.