Whether for learning Chinese or English, the Plateau exists. I've started thinking about this because recently several students I know have have been on about acquiring more and better grammar and vocabulary and I've been telling them naw, at your level it's all about function now.
I've been saying to these students in the beginning there was vocabulary and vocabulary had to be acquired, and then there was grammar and grammar had to be if not mastered then at least sometimes properly deployed. Now, say I, you want to focus on function. I've been claiming a naive idea of function - it's the purpose of your utterance in a given communicative context - and while there is one function, there will be many grammar and vocab options for the expression of that function, all of which will be at least okay, and some of which will be more or less nuanced.
What I've been saying is there comes a time in every language learners life where they have acquired a functionally complete subset of the language and whenever they want to say something (or write), they'll have a handful of relatively effective options and they'll be understood by whoever they're communicating with. According to various studies I've heard tell about, this is where most second-language learners stop actively learning. That is, for every second language in the world, most people settle on intermediate as good enough for their purposes.
So what moves you further?