IMO its reasonable to require that users have close to the most up to date software that their devices support. So its fine to kill off browser support after a year as long as the site can still be accessed on a 10 year old bit of hardware.
Once a feature has been out on all major browsers for a year than its fair game to use. Its also not wasting your time trying to develop a modern web app which has support for netscape navigator.
Going off the usage stats of the site I work on, almost everyone is on the latest version of their browser and the ones that are not always end up upgrading after a warning notice is sent out about old borwsers.