Well these are some really good points! I didnt think about batteries degrading but as far as I know, the more expensive phones tend to have larger batteries which will still get you through a day without charging…
Talking about accidents, I dont think the warranty replaces accidents where you are at fault (?). My phones (cheap or expensive) always had a case and tempered glass on them and I have yet to damage any of them but I get your point, expensive phones have to last for longer. I personally worry more about how long I am going to get security updates though…