Only Indians would consider China their mortal enemy. Frankly, if South Tibet weren't legally Chinese territory, most Chinese wouldn't even bother with it (this might be part of China's strategic plans).
Even if China one day no longer needs Russian resources, it doesn't want Russia to become its enemy. Therefore, maintaining friendly relations with Russia is the wisest choice. You can't try to dismantle Russia just because you think it might be a threat. Doing so will never bring peace; pursuing absolute security in the external environment will only lead to war.