The article you linked answers most of your questions.
- Relative global upstream traffic went down, but not due to other file-sharing protocols but entirely different applications
- I2P is not mentioned anywhere in the article, nor any other sharing alternative
- VPN is mentioned as a potential reason for not being able to identify torrent traffic; VPN has become much more prevalent and promoted in the scene
- The article says, in piracy, streaming websites are much more popular now
It has not been surpassed by another protocol. The relative numbers don’t say much about absolute numbers or usage.
And 10 % of global internet upload is certainly no irrelevancy.
If you buy the book but listen to it as an audiobook, wouldn’t buying the audiobook in the first place be an option then?