No, it's not difficult to support oldIE. Single Column, nothing fancy. But the business people don't want that. They want it to look identical in every browser, and the web designers are scared shitless of losing money to tell them different. The problem is that business people don't know that hours of extra work are required to do this. And when they are told they don't care.
There is a simple solution for that, add it to your bill / launch estimates. Every client I have had always understands when you say "It'll cost X to support that, or it'll bump your launch date 2 weeks". They may not understand the technical details but they will understand when you put it in their terms and ask them "Worth it to you or not?"
Definitely. Something as simple as a bookmarklet can become hell with all the IE-specific rules for cross domain messaging, third party cookies and the like. We're still trying to be IE-compatible but man it does take a hell lot of time.
There's different versions of jQuery. If you require a version that supports older browsers, you might not be able to use the bleeding-edge version. The current libraries will continue to work as they always have. There's no reason for newer versions to be limited by outdated browsers. Developers are free to choose the version they want to use.
New versions do not need to be limited by old browsers. Why not build a mutable API? Your browser doesn't support canvas? Oh, too bad, but the Canvas module will just be undefined so you, the developer, can choose what to do with your user.