Maybe in the early-aughts. Me, I'd be putting my chips on a Webkit future.

Maybe you've had different experiences, but I've had very few problems with forward compatibility in all of the major browsers (IE/FF/Chrome), and I don't know why that would change going forward. Actually, I'd go so far as to say that if you develop your apps to work cross-browser, they're more likely to work in future versions of IE than if you specifically target your dev at a single version of IE. Also, just because I do 90% of my dev work where there are good tools, doesn't mean that I can't or don't test in IE 8- (depending on the target audience).

That doesn't make any sense. Forward compatibility is not an issue at all with modern browsers, and you're not obliged to update either.

Having something that will work 5 years after your consulting gig is not a problem - avoid browser specific features and try to stick to the HTML spec as close as you can.