Easier said than done.
Preservation of national sovereignty is one of the touchiest issues in modern Chinese politics.
This has to do with Chinese history. For China, the modern era essentially began when British gunboats arrived in 1840 and forced the Qing dynasty to open up its ports to the opium trade. There followed a series of conflicts in which China was progressively humiliated and ever more dominated by foreign powers - the Europeans and Japanese. China had to allow opium in, had to exempt Europeans from Chinese law on Chinese territory, and had to give the Europeans and Japanese enclaves and spheres of influence within China. The British took Hong Kong, the Germans took Jiaozhou Bay, various countries took parts of Shanghai. When Chinese peasants rebelled against foreign influence and started killing Europeans, a eight-nation army landed at Tianjin,[0] marched to Beijing, burned down the Summer Palace, and forced China to pay a massive indemnity. After the Chinese people overthrew the imperial system, the new republic was, in fact, controlled by various local warlords. The Japanese progressively took more of China, before launching a full-blown invasion of the rest of China in the 1930s. A core goal of both the Nationalist and the Communist parties was to re-establish Chinese national sovereignty.
What put an end to all of this was the end of the Chinese Civil War and the establishment of the "New China," as the Chinese call their post-1949 state.
Because of this history, the issue of national sovereignty is incredibly sensitive in China. It's easy to say, "China should just give up its claim to Taiwan," but that would be an incredibly explosive suggestion in China, especially if it were something that China was forced to do under outside pressure.
Finally, I'll add that in China, the ROC is not seen as a separate country from the PRC. Taiwan is seen as the last redoubt of the old Chinese government. This is also how the ROC understood itself until fairly recently. For most people in China, asking them to cede Taiwan is asking them to cede part of China, which is a non-starter.
0. A portrait of soldiers from the eight nations (plus Australia and the British Raj, minus Russia): https://en.wikipedia.org/wiki/Eight-Nation_Alliance#/media/F...
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