There are many reasons for this, but I believe "it's what the people want" is not one of them.
You are entitled to your opinion, and it's great that you're able to lay it out in a civil, non-argumentative way. Let me explain to you my reservations about Taiwan becoming an SAR.
Maybe it's my upbringing in a very conservative American household, but I believe democracy and human rights are in fact all they're cracked up to be.
When Chinese dissidents stop disappearing for exercising their constitutional right to free speech; when foreign journalists are not labeled persona non grata for telling the truth; when the state stops literally playing the part of god by putting its hands all over church affairs; when average people don't have their land or other property expropriated with little to no explanation and little to no compensation; when the government is no longer actively preventing its people from learning about what's happening in the rest of the world; when education is not primarily a tool for either economic growth or political propaganda; when disputes other countries over uninhabited patches of sea do not spark government-condoned riots against foreign companies; when the media keeps officials in check rather than empowers them; when citizens are allowed to leave their country at will; when leaders are answerable to their people; when the justice system is just; when the government is legitimized by more than just economic growth which it may not be able to keep going forever...
Then I would find unification to be a very agreeable choice. I may even, at that point, be in favor of it. But that's a far cry from the way things really are in China.
These are all true. But couldn't this same exact argument be put toward the founding of a Republic of Taiwan?
Alternatively, you could say: