I think the analogy is a bit misleading here. The main point you are trying to make with your analogy, is that the drive-thru wouldn't make it 5" if 7" has 75% of the market. Which is true on itself, but it wouldn't make that decision based on the 'only' 25% share (which is quite big, actually), but because of the trouble adjusting the drive-thru, while there is no benefit whatsoever in it (since one looses the other 75%).
To make your error more clear: imagine I repeat *all* that you said in your analogy, but switch the 5" and the 7". Then your end-question becomes: "Do you build it to 7' just because some international body (and 25% marketshare) said that was the "standard" or do you recognize the REAL standard and build it to at least 5'?"
Well, yes, if you are smart, you do. that's because a 7" drive-thru will also accomodate a car that is smaller, while a 5" obviously won't do for the 7" cars. Therefor, making a 7" drive-through will be fine for 100% of the market (in the given example), with minimal extra costs for the owner. One would be a complete fool to not do it.
Thus, as one can see, the answer to your question isn't really based on what standard is being proclaimed, but by who you give the 7" too, and the possible benefit one would get if you make your drive-tru bigger or smaller.
In ALL cases (as long as it's worth it), the 7" one would be preferred, so the analogy doesn't really say anything about what's better: to choose a set standard, or the 'real' standard.
In fact, with the browser(s) it's mostly the same as what I said; websites that can adapt their website to accommodate both browsers, would be foolish not to do so. And, exept for some special cases, most websites DO support them. Will webmasters make their site ONLY available for IE, if they can make it available to firefox too and gain another 25%? I very much doubt it.
Your analogy only works if you accept the premise it's 'IE-cars' that has the 7" (but that's arbitrarily chosen), and that it is impossible for the drive-thru builder to accommodate for cars of 5" and 7" of clearance.
Both premises are, when compared with browsers, highly doubtful to be true, and thus the analogy fails due to those differences.