The Cantonese dialect originally came from the Guangdong/Guangzhou area (Guangdong was originally pronounced Canton, hence the name Cantonese), Hong Kong was originally a fishing village of ~7000 people (so was Shenzhen 40 years ago) before becoming a colony so the language won't disappear.
People from Shenzhen speak Cantonese as well and Shenzhen is already connected to Hong Kong by subway. This Greater Bay Area is really just making something that was more unofficial official. Shenzhen is part of Guangdong province while Guangzhou, the capital of Guangdong is also about 45 minutes by train from Hong Kong. The Greater Bay Area is really more about making it more official that Hong Kong is related as a group with Guangdong.
People from Shenzhen speak Cantonese as well and Shenzhen is already connected to Hong Kong by subway. This Greater Bay Area is really just making something that was more unofficial official. Shenzhen is part of Guangdong province while Guangzhou, the capital of Guangdong is also about 45 minutes by train from Hong Kong. The Greater Bay Area is really more about making it more official that Hong Kong is related as a group with Guangdong.