The firewall and aggressive social repression in general is a red line for China, much like their claims of owning Taiwan and the inevitability of annexation. The dictatorship that runs China - both the remaining old party system and the new system run by Xi - can't exist without the firewall, it's off limits accordingly.

China won't come to the table if you put the firewall in the list of things to be negotiated, that's why it so rarely comes up. For them what that means is, they'd rather suffer any external consequences in trade & economy, and the domestic suffering that passes on to the people, than give up their reign.