That is not completely right. People switch back because most of their contacts are using QQ and WeChat.
And that's actually why people keep using Tencent product -- because all your friends are using it, your mom using it, your boss using it, so you have to. In fact, I think most of those IM's gaining their user base because of this.
Also, encouraging Chinese people using domestic services is good for government, as they had better control of those services. So they did, by selectively making foreign services unstable.
Censorship is not just a mean of control, it's also a business: Government wants control, company wants money, so they formed an alliance to get what they want.