Need for "-ts-mode" modes

[Philip Kaludercic]

The issue has been discussed before, but I failed to understand the
point of duplicating a lot of modes, or creating new modes that depend
on tree-sitter and that don't work or even try to provide some fall-back
if the library is not available.

Or the re-phrase the question, why can't tree-sitter support be
implemented by extending `define-derived-mode', ideally in such a way
that can be translated to some kind of font-lock rules for basic syntax
if the library is not installed.