I so agree with you.
I think most people don't really know what they are on about when they say things like that, they just hear or read the same thing from somewhere and repeat without applying it to themselves or their own experiences.
A gwelio friend who spoke Mandarin pretty well stayed with me for a week and 90% of the time they replied in Canto or looked at me for help, it wasn't that none of them understood Mandarin but just didn't feel comfortable using it or just didn't like to.
Even if Cantonese is a dying language would it really be in my lifetime? If not why would I care?
Also I think that even if your accent is really good, seeing a white face speak a chinese language is *sometimes* too much for *some* people to take in, so their initial response is to ignore it. This happens to my partner a lot even when what he is saying sounds correct.
Definately learn what feels right, not just what is 'in' right now