I made a comment to a friend over the weekend of 12/18 about being Chinese, someone also of Canto background. If the central government of China is every successful in stamping out Cantonese I think I'd have little reason to identify as Chinese in any way. I do identify as Cantonese with respect to my heritage, family lineage, family history, and family culture, so if Cantonese were to disappear, what link would I have to a dominated people that has become culturally extinct?
What sort of genetic/genealogical relationship really exists between the different peoples that make up the Han ethnic group? There are significant differences in appearance, over a range. Even if all of the Han peoples of China are part of one larger group, there is no necessity for them to have the same language. That is the totalitarian impulse of a centralizing political authority. As an identity "Cantonese" expresses more of the specificity (though it may not be the most refined one, as Chinese identity would be specific to village and clan) reflecting subjective reality than just "Chinese."