This will be fascinating to see in practice, with ghostty for example shipping these changes! I expect that the concern you have here will largely be for naught, with some exception. What are some terminal apps you think might be affected, what are test cases?
I didn't read in fully, but what I was thinking in my head is not that we would just totally replace the rest of the colors with arbitrary palette. But that we would sub in better versions of the palette that also used user colors as the base. What was magenta is derived from what the user picked from blue and red.
There's always been such tension between design/creative and users. Apps & designers want their own brand identity, want creative control to make things just so. And are willing to throw user preference & desire on the pyre to get that exacting control. Personally that was always rubbed me extremely the wrong way; I would way rather allow some weirdness & funkiness in, if it leaves the user in control. But I understand the risk aversion, understand the Murphy's law corporatism that makes people and companies want to build strong laws that forbid anything but strictly approved systems, for fear that things go wrong. I understand. But I also things that's a dogshit world to live in.
I have a bunch of Vim colorschemes under my belt.
0-15 are, as I said, a minefield because they are user-customizable: there is no guarantee whatsoever that my user's 1 will be the same dark-ish red as mine… or that it will be dark-ish… or that it will even be vaguely red-ish. It is actually somewhat fun to design colorschemes within those crazy constraints but oh well.
On the other side of the spectrum, truecolors is a nice idea in principle but support is still spotty and inconsistent. In theory, this gives me, the designer, full control over the colors used in the UI, which is a good thing for us and for my users. In fine, if I want my colorscheme to be usable by most users, then I can't blindly rely on this.
Which leaves me with 16-255, which are more widely supported than truecolors and, more importantly, dependable. They have problems, as mentioned in the article, but their _fixed_ nature gives me confidence that the background color of the status-line, for example, will look exactly the same -- and exactly how I want it to look -- in all my user's environments. Which, again, is good for my users and for me. Losing that confidence is what worries me, here.
Like you said, maybe 146 will still be a muted violet —— just not exactly the same -- but I'm not sure about this and I think that, at the minimum, this "feature" should be put behind a checkbox/flag.