But you're describing Emacs. Emacs figures out the language and loads the appropriate sub-modes. It even calls them modes. Modes are mixable.
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.
> Emacs figures out the language and loads the appropriate sub-modes.
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.
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.