I think losing the North Korean buffer isn't something they are necessarily completely stalwart on anymore. With Russia no longer as likely an ally (keep in mind that these days, both Russia and the PRC have troop buildup on their shared border), defending the Korean peninsula no longer possesses the prior ability of a natural bottleneck. Additionally, the PRC may wish to give a little ground to the United States for some extra leniency dealing with the land disputes with India.