While theoretically true, I can't find recent examples of this happening with zero-click exploits on iOS or Android. Without evidence of this being a common infection vector it's not, in my opinion, enough reason to encourage people to get rid of a working phone just because the security backports might be a bit lacking.
The more important security reason to keep up with the latest OS version is the sandboxing improvements that iOS and Android make with each update. If you assume the device will be compromised with a malicious app at some point, you want to have more protections against the malware stealing data from other apps. This is (for now) a bigger deal on Android, where malware routinely makes it into the official app store and malicious APKs are floating around all over the place. But it's worth considering on iOS too, especially if you run a lot of apps from companies that hate privacy or if iOS later allows some form of sideloading.