Really people didn't like XHTML because they didn't want to close their elements. That's it. And now most web pages don't even parse in an XML parser. What elements are standard and which are not is completely arbitrary, what the browser does with them doesn't really matter either. What matters is that you can extract the data from it, if you know the structure (either by following some standard, or by having out-of-band documentation). In that respect JSON and X(HT)ML are similar, except now you can't scrape web pages with a single GET request from the canonical URI, and instead need to run a fully fledged browser that parses javascript and/or read some site specific JSON docs (if any are provided at all).
> because they didn't want to close their elements
That is an incredibly uninformed view of early 2000s web content authoring. The reason people didn't like XHTML was that it provided no tangible benefit.
In fact, my experience was quite the opposite: technical people loved XHTML because it made them feel more legitimate, they just had to sprinkle a few slashes over their markup. Validators were the hot new thing and being able to say your website was XHTML 1.0 Strict Compliant was the ultimate nerd badge of pride.
But these same people didn't use the XHTML mime type because they wanted their website to work in as many browsers as possible.
> And now most web pages don't even parse in an XML parser.
Again with the nostalgia for a past that never was: most web browsers never supported XHTML, they supported tag soup HTML with an XHTML DOCTYPE but an HTML mime type. Why? Because they supported tag soup HTML and only used the DOCTYPE as a signal for whether the page tried to be somewhat standards compliant.
