My wife is Chinese, and I've visited her family who live around Shanghai.
Her parents and grandparents only speak Shanghainese, and thus can't read or write. This isn't because Mandarin is "too hard", but because their generation never had a proper education, and they don't really encourage the elderly to reeducate themselves.
My wife's generation grew up speaking Shanghainese, but then learnt Mandarin at school.
The new generation of kids know enough Shanghainese to understand their grandparents, but they don't want to speak it. Their parents commonly speak Mandarin, and they're being taught Mandarin from the earliest levels of school. Within a couple generations it wouldn't surprise me if Shanghainese becomes an endangered language.
The government seems to have a very successful campaign to move everyone to Mandarin, and they are also actively trying to prevent English from penetrating too deep into Chinese culture. If China can maintain its position as a superpower then Mandarin will definitely become a necessary language for international business.