Any reasons to stay on a credit card? Or is it better to switch credit cards every few years regardless of miles on card?

Some of the Moneysmart and Singsaver credit card signup bonuses look attractive for the spending that I am likely to do. I have no issues paying off debts, but the card I am on has some miles.

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Thinkinig if holding onto those miles or if there are other reasons not considered to hold onto a credit card.