Yep. El Salvador must plan to sell their Bitcoin someday. But I'm not sure if they will try to sell all their Bitcoin. I assume they may gradually sell their Bitcoin, they will also try to have their Bitcoin reserve as well. Sure, it won't be the last day of Bitcoin industry when El Salvador begins to sell their Bitcoin. It is just a normal activity in crypto market. 
As of now, it doesn't seem possible to happen any time soon.
They have a big debt and they have the option to make bitcoin more mainstream instead of selling it.
They could use it as a currency, so if they needed to sell, they could liquidate with other assets instead of liquidating with USD etc.