"Body" is a pile of elaborate biochemistry. The muscles don't somehow evaporate when you stop exercising - it's the processes of the body itself that trim the "excess" muscle tissue.

And if it's the body doing that, you can, in theory, find a biochemical way to make it stop doing that.

Seems like a very broad statement. Do you have anything to confirm this opinion?

"Physical damage and weakness can’t be stopped by a pill."

If you rephrase that to correct English then it would make sense. We aren't trying to stop physical damage or weakness we are trying to prevent it from happening. Pills can prevent many things that cause this.