Is perfect accessibility possible? That is a question I'm asking myself from time to time. I don't think it is possible. I have written about perfect markup, but several days later, I concluded that markup is all about personal opinions. Probably that is the same for accessibility. Once you have added a single graphic to your website people will experience it differently. One might not be able to read it with 256 colors, one might find the alternate text confusing and another could have mixed up his colors.
There should be a way to group people and find solutions for those groups of individuals, but that isn't possible either, since everyone is unique. Besides, you would have to come up with lots of solutions. The best way is to design and develop a site that comforts everyone, but you can't guarantee they get the same experience.
At mezzoblue there was this off-topic discussion about the WAI guidelines. I said that triple A isn't something you can achieve. It can be your/a goal, but you can't get it. One of the reasons is that the current possibilities we have in designing are too limited. Take Scalable Vector Graphics, it is unsupported and quite difficult to have within a document (resizing it, defining height and with, etcetera). Besides, it isn't supported :-)
SVGs are necessary since you may not specify sizes in pixels according to the guidelines (he, it's not my work). Of course I see the point behind that guideline, though I think they shouldn't give the author of documents all the responsibilities. Opera has got some 'page-zoom' option which solves this problem basically. Some aural browser could just take the document structure (valid XHTML) and format it the way that is best for an aural user, wiping out a calendar table for example.
The latter part of your post has been done to death, and I've already said more than enough myself.
I'll comment on the former part, instead, if I may. I think in both cases (mark-up and accessibility) there is no need to understand things in quite that way. The danger is that, having concluded (correctly, in my view) that accessibility is not an "exact science", people throw their hands up and say that it is no more than "opinion".
With an activity like Mathematics, there will be a right and a wrong answer. But consider an activity such as literary criticism. There can't be a definitive statement about a book. (There is also a lot of bad criticism about, and worse many bad theories about literary criticism.)
But that doesn't mean that allwe have is opinion. Nor does it mean that I am only voicing a mere personal opinion if I say, for example, that Henry Fielding is a better writer than Jeffrey Archer - and that if someone said the opposite it would be just as valid. (People have concluded this, but have not given any persuasive reasons for saying so.) So long as I can point to features in the texts I'm talking about and give reasons for saying what I've said, there is a "something" that must be answered, if someone wants to persuade me otherwise.
Similarly, so long as I can point to features in a site and give reasons for saying why they are helpful for access, we've got something more than mere opinion.
I don't believe there is any human activity that is purely technical and that demands a technique but no sensibility. But some people certainly write as if they thought accessibility did. And the consequence is that such people, when they look at other people's sites, try to put them on a bed of Procrustes. I suppose you could say they have got reasons; but they are bad reasons, and it is usually not difficult to show why.
I disagree slightly. My site was, at one point, compliant to the triple A specification; every last remark and flippant guideline. It took a colossal amount of effort
The real problem is keeping it there. Too much work, for next to no gain.
It depends, of course, on what you mean by perfect accessibility. If you mean accessible to anyone regardless of anything, then it's impossible. When it comes to users with disabilities, there is just too big a span to accommodate everyone. Person A with a severe cognitive disorder may require an all-graphic site, because he cannot cope with text. But that makes it inaccessible to person B, who is blind, and needs text that her screen reader can handle.
When it comes to physical devices, you could perhaps come closer to perfect accessibility, at the price of a very boring design, probably. Image replacement techniques and graceful degradation goes a long way, and I think we shouldn't strive for 100%. It's not feasible, at least not with today's technology.
I'm not sure where you've got the idea that the WAI guidelines prohibits using pixels for size, though. They recommend that you use relative units wherever possible (like for margins, column widths, etc.) but they don't say that you have to use relative units for something that has a given, absolute size. If an image is 320x240 pixels, you can say so. Trying to use ems or percentages will just cause the image to look bad in any browser.
I think there are two main problems with designers vs. accessibility:
As I said, perfect accessibility is impossible. Using valid (X)HTML (strict doctype, preferably) and valid CSS, will get you a long way, though. I've found that AAA isn't out of reach, at least not for reasonably static pages.
I more-or-less agree with TOOLman (especially the point about static content). Attaining AAA nearly always ends up being a judgement call by the author.
Most sites I see labelled with AAA certainly are not, or at least in my own opinion regarding interpretation of the WCAG they tend to have minor failing.
In either case the higher up the level you go your returns tend to diminish; wayback in 1999-2000 I made my first commercial AA site for an agricultural contractor. Though a balance has to be struck to obtain the right mix for your perceived audience.
I totally agree, that always there is an opinion possible. And off course, what is the goal? Books aren't good for everyone too. They are useless for blind people. For that reason some books are translated in a point-based language.
The main purpose is to make your visitors comfortable, it seems! But isn't the main purpose to give information, and try to make this accessable for everyone?