> Why not use it for the "tch" sound? (Which, btb, is different than the "ch" sound.)
What are you thinking of? There is no difference between those things.
But your major point here is correct; on the fundamentals there is no reason for the English alphabet to feature a Q.
> "C" is about the same -- by itself it always sounds exactly like "k" or "s". Why not use it for the "ts" sound?
With the modern alphabet there's no reason for a C either. However, the answer to "why not use it for the 'ts' sound" is pretty obvious - that sound isn't part of the English phonemic inventory. It occurs, but that is almost always just a result of what is supposed to be a bare /t/ being followed by /s/ for grammatical reasons. (For an example of the general feeling here, note that an English word cannot start with /ts/ at all.) Why would we use any letter to represent the "ts" sound? We represent it the same way it exists in our language, as a sequence of two unrelated sounds.
> So in pinyin, zh is just like ch, but voiced.
Technically the only voiced consonants in pinyin are m / n / ng / l / r. I think a voicing contrast was present in Middle Chinese, and there's one today in Shanghainese and presumably other Wu dialects, but not in Mandarin.
What are you thinking of? There is no difference between those things.
But your major point here is correct; on the fundamentals there is no reason for the English alphabet to feature a Q.
> "C" is about the same -- by itself it always sounds exactly like "k" or "s". Why not use it for the "ts" sound?
With the modern alphabet there's no reason for a C either. However, the answer to "why not use it for the 'ts' sound" is pretty obvious - that sound isn't part of the English phonemic inventory. It occurs, but that is almost always just a result of what is supposed to be a bare /t/ being followed by /s/ for grammatical reasons. (For an example of the general feeling here, note that an English word cannot start with /ts/ at all.) Why would we use any letter to represent the "ts" sound? We represent it the same way it exists in our language, as a sequence of two unrelated sounds.
> So in pinyin, zh is just like ch, but voiced.
Technically the only voiced consonants in pinyin are m / n / ng / l / r. I think a voicing contrast was present in Middle Chinese, and there's one today in Shanghainese and presumably other Wu dialects, but not in Mandarin.