The benefits outweigh the cons. It's not planned obsolescence.
It means Apple doesn't have to mess around with patching old versions for security updates unless that version was an iOS device's last, and even then rarely. Since downgrading is impossible, jailbreaking devices becomes harder and that is at the root of many exteme exploits. So, security is one reason.
It helps to push the adoption number higher, which is good for developers and users alike because it keeps the hardware and software diversity among active devices low. Much easier to build software with fewer variables and always up to date mechanics.