You can expect anything. It’s reasonable for me to expect my wife is going to have dinner cooked when I get home. That doesn’t mean it’s going to happen.I think it’s a reasonable expectation for an iPhone to last for more than a year and a half, otherwise they would not sell much if at all. If it did indeed fail without user error, then it is not good quality. You will see an iPhone 6s in the wild and you’ll see a 2006 Toyota - the warranty was over a long time ago.
The majority of iPhones are not going to fail right outside of warranty, but this is a possibility. Apple doesn’t offer AppleCare for no reason. They don’t set the warranty at 12 months for no reason either. Some devices are going to fail outside of the warranty period. It’s great to have expectations, but ask anyone whoever has had expectations if they’re always met.