From a pragmatic point of view, even if we were to accept that Mandarin is of greater importance than Southern Sinitic Languages economically and sociopolitically in Southeast Asia, preservation of Southern Sinitic Languages should still be preserved because:
The phonologies of Southern Sinitic Languages are closer to that of Middle Chinese 中古漢語。The most notable common feature is that most SSLs preserve the Entering Tone 入聲, where Mandarin had lost it completely during the Yuan Dynasty. This is why people with better command of SSLs tend to have better mastery of Chinese Characters 漢字 than people who only speaks Mandarin.
Chinese loan words in Korean and Japanese also preserve many features of Middle Chinese 中古漢語，especially the Entering Tone 入聲. SSL speakers usually find it easier than Mandarin-only speakers in learning Korean and Japanese, giving us greater access to these 2 High Cultures.
Regions that preserve the Entering Tone 入聲 tend to have better economy, such as Taiwan, Hongkong, Guangdong, Shanghai, Zhejiang, Korea and Japan. While we cannot confirm the direction of causality, the correlation is strongly evident.
There are some preliminary studies suggesting that speakers of SSLs have higher overall IQ than that of Mandarin-only speakers. This may be due to the relatively more complex phonologies of SSLs which serve as intensive brain training to their learners.
