I guess the reality just isn't so rosy.
Even if all it was supposed to do is syntax highlighting, different languages are usually highlighted differently and presumably the person maintianing the bindings would need to be a language expert with strong opinions. The options are they either maintain their own mode or a module in `treesitter-mode` which is basically 2 ways of describing the same situation.
Although to address your other point it is quite funny that after 40 years and 29 versions of experimenting in OS design, GNU Emacs is starting to implement syntax highlighting properly for its system text editor.
We're in a transition period while everything is rewritten to use tree-sitter. In a few years all the default major modes in Emacs are probably going to be tree-sitter based - unless the maintainers believe them to be better than tree-sitter.
Not really. You setup hooks, usually based by fileextension, and emacs execute the hooks. Emacs itself has no understanding of languages. And this will not work well when there is no hook executed.
> Modes are mixable.
You can load multiple minor-modes, but there is always just one major-mode per buffer, and languages are usually major-modes. But ok, treesitter can be handled as a minor. But it still needs explicit support. You cannot give treesitter control over a range of text, let if figure out the language automatically and let it make it's thing. This needs explicit support.
So, the line of the joke goes, it must not really be a text editor because it is not very good at it and has a wild array of other capabilities. Like "M-x dunnet". Must be an OS.
Of course the reality is that Emacs just copies good ideas from other people. A pretty normal Emacs setup uses Vim keybindings, LSP and now tree-sitter to get a good experience programming.
Too bad other people don't copy good ideas from Emacs. Why other text editors/processors have no equivalent of view-lossage is a mystery to me. Also, where-is, describe-key, insert-char, transpose-chars and lots of other very useful stuff...