I have not run i2p or otherwise paid much attention to the project for more than a year, but I did notice it had a remote code execution vulnerability discovered last summer or so (stemming from its default-on web admin interface: http://blog.exodusintel.com/2014/07/23/silverbullets_and_fai...). Just because of that I would not consider it an alternative to Tor. However it's not true that "nobody uses i2p". The network is somewhat lively. Another thing is that relay mode is the norm rather than the exception (unlike with Tor), meaning your relay going down can be correlated with your i2p torrent client going down etc.

I'd look to GNUnet (https://gnunet.org/) as the future. It is more than just a Tor alternative, since it also has non-realtime features. Unfortunately it also has few or no users. A neat feature for any aspiring network would be capability to run over Tor, so you don't start with an empty anonymity set.