If Taiwan declared indelendence tomorrow and PRC didn't do anything how would life of average citizen is Beijing change?
The prices of pork and cabbages would largely remain the same, I'm pretty sure. Subway will still run on time. If that's what you ask.
Other than, everything would be different. It's called national sovereignty and national dignity. If this is very difficult to understand, here is an analogy that might be helpful: if the European and American think "freedom, democracy and human right" are so damn important that it's worth dying for, then Taiwan for Chinese is at least million times more important than those values for European or Americans.
I hope this explanation goes a tiny step to help you understand how important Taiwan is for the Chinese.