The problem is that if you don't update your software, you can't get the latest security updates (it would be too costly for manufacturers to have to support security updates for multiple OS versions for the same phone).It's about time people are not forced to update to the new software, or at the very least be able to revert back to the original OS.
Without the latest security updates, you are vulnerable to having your phone compromised in worldwide mass attacks that are increasingly common. Not only would that harm you, and drive phone costs up when you have to call your manufacturer or carrier for tech support when it happens, but furthermore it harms other users when your phone is hijacked as a bot to participate in further attacks.
And problems with updates are actually rare. It's just that most people don't run to the forums to ask why everything is going smoothly for them.