You're undermining the fact that to build services of that scale is not trivial at all, and the fact they have managed to do it is the basis of why they are an effective competition to SV, now that they are as economically strong as the Bay Area is.
Once they are done with "replicating" SV, they'd start "inventing" too. What's stopping them from that?
I'm not undermining them - simply stating that there was a vacuum created by the lack of competition. Even cloning successful services at scale is incredibly difficult, but it's not 0 to 1, it's 1 to n. Nonetheless, I agree Chinese startups are using their current leverage to innovate in really interesting new ways.
I highly doubt there is a "lack of competition" in China. For each and every one of those "Facebook, Messenger, Instagram, Twitter, Snapchat, Youtube, Gmail, Dropbox, Vimeo, SoundCloud" you mentioned, there are a hundred similar companies.
In fact, I'd argue that competition is rife and more ruthless in China than in Silicon Valley. The moment you have a good idea, ten companies will appear the next day hot on your heels. Even if the American stalwarts were to enter the Chinese market, who's to say they won't be crushed by Chinese competition?
With that said, reputation and word of mouth is everything in China. Before an app like Didi was the king, people were changing ride share apps every month. New deals, new coupons, new rumors, etc... The tide changes really fast here. Didi just happened to be the only app that could hang in there.
Regardless of how many people there are, if 10 people online say something is not worth using, no one else is going to use it.
Once they are done with "replicating" SV, they'd start "inventing" too. What's stopping them from that?