I think Cantonese will remain a minority language.
Mandarin is the majority and is simpler. The reason I don't think it will get swallowed up by Mandarin is because the Cantonese culture is very strong and distinct, especially in and around Hong Kong. Also if you ask foreigners about Chinese people and culture they will point to something or someone that is Cantonese more than half the time. This is mainly because Cantonese people have been the established face of China to foreigners as they were the first major group to settle outside China.
I personally like Cantonese culture and language more than Mandarin and the other cultures in China( will probably get a lot of hate for that). Anyway I was thinking how can Cantonese grow? It could become something like Quebec in Canada or Catalan in Spain, which wouldn't be a good thing.
Had my first trip to China last weekend, and I was taken by surprise how many people spoke Cantonese. Granted, we did go to 廣東, but still it was a nice surprise.
Anyway, as for Cantonese survival, I believe there is a good chance since Cantonese people can be very stubborn when it comes to their culture and language. However, I believe places like HK and Macau need to stop teaching Chinese-based subject in Mandarin.
Although there is no official writing system for Cantonese, it is used daily in casual correspondence. Standard Chinese is used for formal situations, like work. That said, before Standard Chinese, the written system and spoken language for Chinese differed vastly.
So long as there is media produced in Cantonese (which looks likely), the dialect will endure. There are more than a few tens of millions of Cantonese speakers all over the world.
The biggest weakness is a lack of formalised writing system for many words. Another weakness is the decline of Cantonese opera.