Ad 1) If C99 (or C11) was supported, you could perhaps use C99/11's features to refactor parts of the legacy code into more elegant code than possible with C89

Ad 1) If C99 (or C11) was supported, you could perhaps use C99/11's features to refactor parts of the legacy code into more elegant code than possible with C89 (and yes, there are sometimes good reasons not to use C++). Ad 2) If there is support for features there will be people using them.