China and Taiwan are already quite integrated. Taiwanese people are able to live and work in China, and Taiwanese corporations operate factories there. The Chinese government has been luring them with tax credits.
Trade between the two countries keeps growing and is vital to both economies.
In many ways, it is the same relationship as Canada/US or Australia/NZ. Cousins who live next door and hang out a lot.
Those countries see no particular need to become one entity, because the status quo works very well.
To invade Taiwan means risking something more important to China – their economy – so that alone should be enough to dissuade them.
When the great split happens, Taiwan will need to choose a side. They will choose the East.