Mandarin is not fit to be the official language, simply because of one reason - it has too many synonyms, compared to Hokkien and Cantonese. (I am sorry, but these are the only dialects I know, hence I cannot speak for other Southern dialects)
I think you mean homophones (same sound, different meaning) instead of synonyms (same meaning, but different words).
I have come to doubt Manchurian influences on Mandarin, but there are still differences that I know:
Differences are not necessarily influences from non-Chinese languages.
1. no s/sh differences in Cantonese and Hokkien.
Not in Standard Cantonese or Hokkien, but there is for Shunde Cantonese.
2. more endings in Cantonese and Hokkien - not only -n and -ng.
True for Standard Cantonese and Hokkien.
4. Cantonese and Hokkien sound strikingly similar, sometimes identical with Japanese and Korean.
Sometimes, but most of the time Cantonese and Hokkien are considerably different, and this can be more than Cantonese-to-Mandarin difference.
For example the -in rhyme in literary Hokkien corresponds to -in in Mandarin but usually -eun/-an in Cantonese.
I am not expert in these foreign languages, but I know that 'books' are pronounced 'chek' in both Hokkien and Korean (compare ce, 册 of Mandarin); 'teeth' pronounced 'ge' in both Hokkien and Japanese (compare ya, 牙 of Mandarin). If they differ from Vietnamese, please say so. I am completely unfamiliar with Vietnamese. However, I am not sure if the Hokkien character for ge is 牙..
Remember that Japanese has 2 layers of Chinese loans, kan-on and go-on.
牙 is ge and ga. but nha in Vietnam.
I have to check again [for 册].
jik, with j similar to Malay c.
They are all pronounced as xī in Mandarin。 The decrease is not absolute, but to a certain extent homonyms are less.
According to my experience, the merger of sh and s somewhat highly increases the number of homophones in Cantonese (not sure about Hokkien).
But 1 thing for sure - the homonyms are less in Cantonese, as shown above. I reckon Cantonese should be the official language, and it is not too late to change... I think.
I think the syllable count is still higher in Standard Cantonese (not sure about the "lazy sound").
Cantonese is hard to be official language because some of the particles do not have a simple way of representation (e.g. ge3 (possessive), go2 (that)).
Furthermore, many non-Cantonese won't be able to pronounce many of the Cantonese phonemes accurately without much training.
Mandarin is good for a "lingua franca"/"koine" because it's simple, but not "that much".
The drawback is of course difficulty in expressing complex concepts.
If I had my way, I'd keep written Putonghua exactly as it is, but make Middle Chinese spoken in the Tang/Song dynasty the spoken Putonghua. That way, the national spoken language will belong to all Chinese but no one on particular. Failing that, the government must follow Taiwan in permitting regional dialects.
Sorry, but Middle Chinese phonology is simply too complex for modern Chinese and therefore it's not practical at all.
Permitting regional dialects as the main language of a TV program is there in PRC, for province-level and sometimes also prefecture-level.
This thread is about the attempt to change this.