Not sure what might break if we do that: the main user of
cl-symbol-macrolet outside of generator.el AFAIK is the with-slots of
eieio, so the question is whether some users of with-slots expect
a subsequent `let` binding to temporarily change the slot's value.
I just checked and it seems that no code in Emacs itself relies on this
behavior, so maybe it's "safe" to change it.
At least the docstring of `with-slots' and the EIEIO manual both state: "Both ‘setf’ and ‘setq’ can be used to set the value of the slot." In particular, it doesn't state that `let' can be used for this purpose. So as long as `setf' and `setq' continue working, I think it should be OK to change the behavior (it should still be documented as incompatible Lisp change though).