Earlier quoted context omitted.
For the vast majority of history, the vast majority of people could not read or write. We also only learn to read or write long after we learn to speak, so it does not seam reasonable to me that more complex alphabet results in a more complex language. Chinese and Japanese have arguably some of the most complex "alphabets" (yes, Japanese has hiragana/katana, but they are more recent and Chinese only uses pinyin/bopom…
How exactly do you figure that Chinese has some of the least complex sounds? Chinese not only has just as many sounds as other languages, they also have the added dimension of "tones" which can be very easily described as "sounds" and trip up most people learning the language.
The number of phomenes in (Mandarin) Chinese is indeed not low, but it has only ~1200 different syllables (including those differentiated by only tone differences), which is pretty low compared to other languages.
English has 26 letters which represents 44 phomenes, modern Chinese has ~8000 characters for 1200 syllables, I don't think you can find a system in there.