There is the law of unintended consequences. Other countries will now make American small business products much more expensive to export. Why would they give a De Minimis to these small businesses when the USA doesn't reciprocate?
You're only thinking about China and not the rest of the World. US small businesses buy things (under $800) from countries like Canada and the EU, and were able to do so because of De Minimus. Canadians could buy things from US small businesses without worrying about duties as long as it was under $500.
Now even the smallest item, even a gift from abroad worth $20, will incur a duty of $50-$250 plus administrative fees from the carrier.