Well, yes. But as FF+Chrome+Safari have more than a third of the marketshare, you can start writing parts of a website that work better with HTML5. For example, an application that uses the application cache to be usable offline, and tell IE users they could have an offline experience by changing browser. This means that everything still works, but people with HTML5 browsers will have a better experience.

Sure you can.

Depends on your target audience. IE has vanishing small market share amongst several groups such as tech savy, non-MS developers, Apple fanboys, etc.

Also, if those capabilities lead to features that vastly surpass your competition it'll be easier to own majority of non-IE customers than it would be going after all customers with generic features.

You don't need every person with a web connection as a customer, you just need enough to be profitable/successful.

You do have a point. But if you're writing web stuff for the intranet or if you otherwise have enough control over your users to control their browser choice then it's a much better use of your resources to code for the future (FF, Safari, Chrome, Opera) than for the past. Big stupid corporations who still insist on writing all internal stuff to work with IE6 are, sadly, still around. But declining.

There will be shims.

http://www.google.com/search?&rls=en&q=html5+shims