No previous definition of theft ever included "so that I have it and you don't" until hairsplitting internet lawyers wanted to be able to download things they didn't own and not be called thieves.
No previous definition? Find any jurisdiction anywhere that has ever defined copyright infringement as theft. It never happened.
If I'm charged with theft, and I prove that the owner retained possession of the "stolen" property, I'll be acquitted. Obviously.
None of this excuses copyright infringement, but you simply used the wrong word.
Illegal immigration is not arson, and copyright infringement is not theft.