From what I've read[1], vim9script was pushed and developed almost exclusively by Bram. With him, a lot of knowledge about its internals and vision for its future dies.
You're correct it was very much a "Bram project", but that doesn't mean the language needs to die with him: other people can work on it (and already have!) Vim9Script is also "finished", more or less, as "finished" as languages get anyway. The features Yegappan mentions are what we might call "optional features".
If I look at some Lua plugins and compare that to some of my Vim9Script (or even "legacy VimScript") plugins then I think /Vim9?Script/ "wins" hands-down; it's just much more convenient for programming an editor. It also doesn't help that IMHO Lua isn't all that great of a language to start with – it's not horrible either, just not great.
On Windows gvim works loads better (even on Unix systems gvim is arguably better, because terminals kind of suck and you run in to loads of graphical and input limitations pretty quickly).
But in general: neovim doesn't offer me anything I want or need, I will have to spend time on migrating (e.g. my vimrc would error out, I need to deal with changed defaults, etc.), and for most things I prefer the "highly compatible" attitude from Vim/Bram, which is a trade-off that's not without its downsides, but I really like it (for most software).
Personally, I do heavy coding and writing in Neovim (with more extensive config, like LSP and Copilot), but I use macvim and gvim (depending on which OS I'm using) for quick edits or file viewing, because I often need to open files from Finder or Nautilus and I don't have a terminal open in that directory.
[1]: https://github.com/vim/vim/discussions/12736#discussioncomme...