As someone new to web design and programming I am failing to see adequate support for the argument that we should write code that is supported by old browsers. I know that we can overcome the faults of old browsers with polyfill and web shim libs but like why are people using old browser versions in the first place? Why not just prompt them to upgrade their browser upon visiting the site? OR why not just NOT support old browsers and try to build a social stigma around using deprecated browsers?
I know that it's not wise to push away customers due to their browser preference but theres a reason why horse and buggy is not allowed on the freeway in the fast lane you know what I mean? I feel like if we keep supporting old browsers we enable the old browser users to continue their terrible choices.
I'm sure that there are some obvious reasons that I'm not seeing and this post is an attempt to lure them out to widen my understanding of the subject. Please comment with any case that you see as evidence for supporting old browsers.
After reading the other answers...
Don't forget about the medical industry. Especially the billion dollar one called Telemedicine. Folks 60+ and older are still using Vista, can't leave their homes and need to have a video conference. This is why, whether current browsers love it or not, Flash is a necessity.
Also, X-Ray software that hasn't been updated for over a decade - and is needed to run antiquated devices - are running on Windows 98 PCs.
Unfortunately, we'll all be there. Running a quad-core i7 4ghz CPU, with 32 gigs of RAM and a 2 TB SSD in 20 years, may not allow you to update to the new version of Chrome - with Super-Duper's Vidirama GoWoah 2.6 - until the hardware is upgraded too. And, if at that point, you're living on social security... good luck with that one.
So, accessibility and backwards-compatibility is much more important than people give them credit for.
Also, don't forget about vendor prefixes. If I were to attempt predicting a "trend gone hog wild" - it will be the requirement of something like Modernizr to become native in the future, because Moore's law is basically explaining to us why we update 36 apps on our phones and wake up the next morning to update 48. Everyone loves to go their own route, without paying attention to how much of an incompatibility issue they're creating within the standards that will eventually evolve even more.
However, I'm a fan of the horse and buggy... and don't mind creating artwork on paper as well as socializing with my neighbors over a barbecue.
It can be many things. Here's what I can think of:
I see three major groups of people who need software for old browser generations.
While you as a developer can always decide that your very own product only runs on a fully functional HTML5-enabled browser with WebGL, WebASM, etc. enabled, other people might chose differently because of reasons important to them. If they pay you, you better do your best to support their decision ;)