>>> This would make sure the `treesit-language-at-point-function` is good
>>> enough to implement the default behavior and it makes it possible to use
>>> `add-function` on `treesit-language-at-point-function`.
>>
>> Unfortunately, it's too late to add a non-nil default value.
>
> Is it?
> I can't find any code out there that seems to care what is its default
> value (all users set it but doesn't look at it, AFAICT).

This case looks problematic:

                 ;; LANG can be nil.  We don't want to use the fallback
                 ;; in `treesit-language-at', so here we call
                 ;; `treesit-language-at-point-function' directly.
                 (let* ((lang (and treesit-language-at-point-function
                                   (funcall treesit-language-at-point-function
                                            pos)))

But I believe this whole 'let*' can be replaced by just
the 'treesit-parsers-at' call.  I hope Yuan could confirm this.

>> I already tried to do the same by adding 'forward-sexp-default-function'
>> to 'forward-sexp-function', but failed spectacularly (bug#70426),
>
> Yes, that one is much harder (it's one of the first I tried to change),

If no more code outside relies on the default value,
the default function could be added like this: