I'm more surprised that people actually expect us to have this in the back of our minds at all times. There's a reason they didn't go replacing all stairs with ramps.
Architects spend a lot of time worrying about accessibility requirements. Accessibility, fire safety, and car parking are the major factors that shape buildings.
Definitely a fact, but I wasn't suggesting that it isn't. Everyone knows there are clear and obvious benefits to having a site that is accessible to everyone as opposed to only the majority.
What I mean by the stair/ramps remark is that stairs weren't suddenly removed from the architect's toolkit when the wheelchair was invented. Since ramps are clearly not used in all instances they are not always appropriate or desirable.
The same goes with web design, which is as much art as is architecture. Not all sites are going to select between a limited range of colors to cater to a relative few if those colors happen to clash horribly with their intended design.
But if you've only space for stairs or ramps, then you'd have to use a ramp, which everyone can use, rather than stairs, which most but not all people can use. Similarly, if you can only have one colour scheme, you ought to be picking one which everyone can read, not just most people. It's not as if it inconveniences non-colour-blind people.
It's not always the case that you would choose a ramp over stairs given the choice of only one or the other. For example, there aren't many two-story homes that have a ramp leading to the second story. (Not to say they don't exist, just that they are a glaring exception.) That's what I mean by not always appropriate or desirable.
I feel I might be getting astray here. I simply think it's odd to impose on--what is as much an art form as it is a medium for communication--the restriction of only being able to use a certain color scheme. I'd be bummed out.
Luckily you don't have to sacrifice any aesthectics. SparkFun does a good job of making their stock status icons accessible by employing a combination of color and shape to help color-blind people recognize the status easier.
I'm sure colourblind people would be even more "bummed out" that your design was unusable. Since, as you go on to mentioned, you don't have to sacrifice aesthetics for the sake of accessibility, I don't see why accessible design would be a problem.
While I take your point that ramps in particular aren't always appropriate, that doesn't mean that accessible design in general isn't.
I've toured an older apartment building (3-4 stories, 1920s, quarter city block) that had a ramp instead of stairs + elevator. It worked surprisingly well.
Ramps aren't actually accessible to everyone - especially for older folks, short steps and flat sections are easier to navigate than a continuous gradient.
Surely in these days of accessibility awareness this ought to be a design goal by default unless otherwise stated?