Even if you display to 70+ browsers you can still get away with programming two versions, as most websites do today (or more than two, as most major websites do). One will simply be less complex and pretty than the other (CSS3, etc).
In the future I imagine that will be the standard: Modern version of a website, and then lowest-common-denominator version, which works with IE[X]+ and older mobile browsers, and stubborn/corporate firefox users, etc.
In my case I don't have a choice: Canvas cannot work on IE6/7/8. Excanvas works until animation is introduced, then becomes progressively worthless.
You know, as a long-time Linux and Firefox user, I've made that point to a number of newspaper web sites, etc etc etc. I usually get the written equivalent of an eye-roll.
Why is it that it's OK for a business to ignore Linux and Mac users (back when Macs didn't have such a huge mind-share). or Firefox and Safari users, but it's not OK for a busines to ignore the same fraction of Windows users?
I don't think it's "OK" as much as it is a business decision. Developing software for various platforms is vastly different (in terms of resources) than developing websites for cross browser compatibility. The barrier to entry in the web world is also much lower, which leads to a higher level of competition and as I mentioned if you don't support a browser, a competitor will. As you no doubt know, that same statement isn't commonly made about platform software.
Developer time spent supporting IE6 is developer time that could have been spent addressing feature requests or other issues impacting the rest of your user base. Or perhaps it could have been spent supporting a platform your product doesn't yet support at all, such as Android or iPhone.
Point is, there's always an opportunity cost for spending developer time supporting any feature or platform. No matter what you do, you're ignoring somebody. There is nothing special about IE6 users in this regard, particularly given that they are now a distinct minority.
Don't assume users have a choice. Even if they do, don't assume they understand the choice.
>why should we care to cater to them?
If you want to ignore a % of your users, that's fine. Some of your competitors wont. That's your choice. Just realize you can be losing potential users/customers and that lesson about updating their browsers will be lost on the customers that went elsewhere.