The Internet saves a lot more on storage and bandwidth costs by not shipping an XSLT implementation with every browser than it does by allowing Joe's Blog to present XML as an index.
For one, in many cases the XML + XSLT is more compact than the rendered output, so there are hosting and bandwidth benefits, especially if you're transforming a lot of XML files with the same XSLT.
Imagine 1000 numbers in XML and a XSLT with xsl:for-each which renders a div with a label, textbox with the number and maybe a button.
That's a simple example. Output would be a lot longer than XML+XSLT.
Because those neglecting it are the same that want to remove it. So it's not “we want to remove it because it's neglected”, but “we want to remove it so we'll neglect it”. This is a pretty standard M.O. for the destruction of the commons.
If you look at the WHATWG GH issue, you'll see that two distinct, modern, maintained implementations of XSLT, one of which in Rust (so considerably less likely to be affected by memory bugs) have been proposed as alternatives to what's currently used in WebKit and Blink. The suggestions has been ignored without a motivation, because the neglect is the point.
Most of the issues of using client-side XSLT is that browsers haven't updated their implementations since v1 nor their tooling to improve debugging. Both of these issues are resolved improving the implementations and tooling, as pointed out by several commenters on the GH issue.
That kind of demonstrates why XSLT is a bad idea as well though. JSON has its corner cases, but mostly the standard is done. If you want to manipulate it, you write code to do so.
JSON correlates to XML rather than XSLT. As far as I'm aware, XML as a standard is already done as well.
XSLT is more related to frontend frameworks like react. Where XML and JSON are ways of representing state, XSLT and react (or similar) are ways of defining how that state is converted to HTML meant for human consumption.
The intuition with the columns of pennies over a plate doesn't translate because in the plate case the surface is fixed (the surface of the plate) so the pressure depends on how much weight you put on it. In the case of multiple columns of water, the pressure you're looking at is the pressure on the combined surface of both columns, which remains constant because both the weight AND the surface are increasing proportionally, keeping the ratio (that gives you the pressure) constant.
