This is a really serious risk, but it is also important to consider that China isn't reallly one country. - Even internal migration is restricted.
If you look at cities like Shanghai, Nanjing or Beijing, there's a subtantial minority already basically living with western living standards. That number is still growing quickly.
Furthermore, China has little problem letting old people work longer - or potentially even implement pretty brutal policies regarding the elderly.
So while China is growing old very quickly, and this will slow down progress, I do not believe it will lead to a halting of progress.