But at least it was --- pre-GPLv3, pre-prompt_toolkit, etc. --- a standard. It would be nice to again have a single de-facto standard line editing system that I could
1) customize once, for all my programs, and
2) compose with other programs (e.g. shells embedding in other programs).
It'd also be nice for it to be friendly to high-latency connections.
But alas, we're heading towards a world in which the terminal is just a GUI with Minecraft pixels.
This has been the case for decades already. Some TUIs use ncurses, some don’t. Some use 7bit escape sequences for switching to drawing characters, others use 8bit control codes. Neither fish nor (and correct me if I’m wrong on this one) zsh use readline.
There never was a standard way. Some de facto standards exist but even these were ignored as often as they were used. And the reason for this is because there never was a standard VT in the hardware days (DEC, Tektonix, etc all did things slightly differently) let alone in the xterm, VTE, et all days. And all of these different terminals have been driven by different operating systems. For example ‘ps’ on macOS uses different escape sequences to GNU ‘ps’ yet they mostly achieve the same output. So imagine how inconsistent things were when you had fundamentally different time sharing systems.