Do I have to update from 5.0.2? Galaxy s6

Hello guys, I am 100% new to Android and I was wondering if I ever need to update from 5.0.2. Because I successfully rooted my device but I tripped Knox. But I don't care because I want the freedom of my android and I am quite frankly not worried about the warranty being void. Someone at Best Buy told me that if I don't update my android version eventually the apps stop working with the version I am on. I didn't flash a rom or anything like that but my device status is custom. So I am wondering if I am able to stay on 5.0.2 forever without ever having to update. Because the guy at best buy said that some apps will stop working, I tried to do a software update through about device but it said that the system has been modified. So what would you guys on the xda forum recommend me to do?

Thanks

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