It's always about the bottom line, whatever the product. I work in the sewing industry, a real thermometer of industry. If it is possible to make good glass in China, I'm sure they would or do?
China is not as you may imagine, it can produce good quality products when it has to. Machines do a lot of the work in any industry. Quality control can be as tight in China as any country when it has to be. Note, when it has to be!! Left to their own devices they will produce the cheapest junk they can, as we have seen many times. A little control and they can hang with the best.
No idea about scope and lense production but, if it can be done, it is being done, I am sure!