No one wants to be locked into IE. I think that's a fair remark. But by browser sniffing, you are locking yourself into the support models for specific browsers rather than doing feature detection and future-proofing your site against ANY browser lock-in.

That said, no one is asking to support a particular browser. If your site is just doing feature detection rather than browser sniffing, you can use an objective measure such as "sorry, this application requires  feature which isn't currently supported in your browser." Instead, there is a subjective test for the browser itself even though it might very well support those features.