Using an iPhone 5s without security updates

Hello. I know this is not a strictly hardware related issue but I would appreciate any thoughts. I am considering whether to upgrade to a new phone for security reasons. Currently I have an iPhone 5s with a very degraded battery but other than that it is functionally fine and am perfectly happy with the performance. However from what I’ve read it’s not a good idea to use any iPhone beyond five years after production ended given that Apple stop releasing security updates, meaning 2021 for the 5s. I am slightly sceptical that this risk is exaggerated and that as long as I am reasonably careful - avoid downloading any dodgy apps or visiting certain websites - then there is no real problem and can safely fit another battery to use for years. So for a complete novice, would you say the risk of using a phone without security updates is legitimate and should go ahead and get a new model or stick with the 5s? Thanks