You're basically saying that it's ok to make any UI worse because users will either put up with it or stop using the feature entirely (which just means that they didn't care about the feature in the first place).
1.) Why is it ok to remove the button - because hardly anybody uses it.
2.) Why does Firefox removing the button not meant that "RSS is dying" - because people who currently use RSS will keep doing so, just without using the button.
Your second point is flawed. The button makes using RSS more convenient. Removing it makes it less convenient. Saying people would not be "put off" by making it less convenient is naive at best.
