I'm a VSCode user as of a few weeks ago for the same reason. Trying to get React to play nice in emacs, especially when compared to what I got out of the box in VS Code, was an exercise in frustration. Similar problems with Elixir.
I really love spacemacs but I just genuinely don't have time to screw around with my editor constantly, so I ported the bindings I found essential from spacemacs, turned on VS Code's vim emulation, and it's actually been a pretty good experience. It's not perfect, but it's good enough.
rjsx-mode has been around for a few years and handles jsx fine. coupled with prettier mode, I don't have any complaints. have you given those two a try?
> Consider the poor quality of the existing js modes for 26.x, any idea why this depends on a future version of Emacs instead of current versions?
You might as well look at the thread js2-mode was revoked[0] and ergoemacs' article[1].
> Alternatively, could somebody convince the author of the railwaycat port (Emacs with better macOS integration) to get a version 27 out soon?
Er... I would like to point out that it's not 'railwaycat' port but more of Mitsuharu Yamamoto's port[2], and AFAIK he has claimed that he has no interest in maintaing an emacs27 version.
Disregarding the history behind js2-mode, if it has been decided that Emacs 27 will feature a much improved js-mode, I was just puzzled that it wasn't put into 26.x as a package already.
I couldn't remember the name of Yamamoto, which is why I said the author of the railwaycat port, where the latter is mostly packaging if I understand it correctly. But Yamamoto is doing most of the heavy lifting yes, and I'm thankful for it.
No. He builds off of stable releases because otherwise something could change in the underlying implementation that breaks emacs-mac. It would mean more maintenance on his part.