As not2b suggests, it's more nuanced than "Hong Kong is just another city by 2047". Firstly, they're supposed to be separate for at least 50 years. The ambiguity comes in with determining how separate the "two systems" really should be. Are they allowed to specify the national curriculum? Are they allowed to extradite criminals?

After the 50 years are up, Hong Kong can potentially continue to act semi-autonomously as an SAR. (It's just not supposed to change until after that date.) Hong Kong's current status is economically advantageous to China, as companies have a place to make inroads to the large Chinese market in a business-friendly environment that typically respects the rule of law. The SAR is also given special economic status by some foreign countries, which would likely be taken away upon 'complete integration'. The integration that you're talking about is going to be a long process, and there won't be a singular point where it's clearly happened. So we shouldn't be surprised that China is making efforts to integrate now, but we also shouldn't be surprised by the resistance.