I'd love to learn more Cantonese, if only because it would make traveling through Hong Kong and southern China much easier. Probably more accurate to say that mandarin is becoming a lot more prominent rather Cantonese fading away.

That said, I can see where the defensiveness comes from. It's true you can get by as a foreigner in Hong Kong pretty decently these days if you have both English and mandarin. You still can't experience the city like a local but for simple communications the people who can't speak English generally understand enough mandarin for you to get by and vice versa. At least that's the case on the island in the city - I don't know about the New Territories.