Is this just for philosophical reasons, or did you encounter a specific issue because of this?
AFAIK, having an ANSI-encoded MessagesFile is still supported, and the LanguageCodePage is required in order to decode these. Bear in mind that beyond the files provided with Inno itself, there may be additional unofficial language files around, as well as application-specific files (where they've chosen to use an external file instead of inlining custom messages within the script). What you're proposing would break any of these legacy files that haven't already been converted to UTF-8.
This also applies to other supporting files like license files and info files, incidentally, which are even more likely to have application-specific legacy ANSI files floating around.
Note that the value of LanguageCodePage is ignored for files that are already UTF-8, so simply converting the language file itself is not a good reason to change it.