"It makes me happy to see my partner happy" is Relationships 101. Just because people gave it a word, doesn't mean it didn't exist - and I'd claim the opposite, poly people had to make a word because its not seen as the default in that community.
This is like saying, I invented the word "hiverchill" which means "that feeling of cold when it snows". You can't say this was a blind spot in our language. We didn't need this word, because of course you are cold when it snows.
"Compersion" is usually more than just "it makes me happy to see my partner happy", but "it makes me happy to see my partner happy, even when that would (at least traditionally) inspire some bad feeling in me."
Compersion isn't the feeling you get when you give your partner flowers and they smile, it's the feeling you get when your partner tells you a story about how nice their date with someone else was. It's a very particular flavor of joy-for-partner, that for some people doesn't exist at all because it is clouded by jealousy or fear or other feelings.
> poly people had to make a word because its not seen as the default in that community.
Feeling joy at your partner going on a date with someone else is not seen as the default in any community except the poly community.
This is like saying, I invented the word "hiverchill" which means "that feeling of cold when it snows". You can't say this was a blind spot in our language. We didn't need this word, because of course you are cold when it snows.