I was thinking like you too for a long time. Why don't China just take Aksai Chin for Zangnan? But after 2017, this is no longer acceptable. Its because the Indian elites don't just want Aksai Chin and Zangnan. They want Xizang (Tibet) too. They have been harbouring and propping up that Dalai Lama and his CIA-made Tibetan separatist "government" on Zangnan for decades now. They have been talking about Xizang as if it is their backyard. Whenever they speak about their border with China, they specifically say that they have a border to with Tibet instead of China. So they have never respected the One China Principle despite their official stance. So no, Zangnan needs to return to China permanently, and they need to capture or exile that Dalai Lama and his separatist gangs for good.
The Indian government and elites can never ever be trusted. They always double speak, and they have trouble sticking to agreements. Give them Zangnan, and China is officially recognizing Indian ownership over the Dalai Lama and ancient Tibetan lands, along with their historical sites. Pretty soon, we are going to hear the Indians start demanding for the rest of Xizang. India had its chance when China was more agreeable, and it went to war anyway. So that opportunity is gone, and should never return.