In any case, just because every legitimate user could be a relay node, doesn't mean the relay nodes can't be Sybil flooded (attacked).
They certainly could, it's just the argument that someone providing relay services for free is suggestive of questionable intentions does not apply to i2p the same way it might for Tor. Most of the time it is simply an indicator they are an i2p user.
I suppose in theory I2P could become widespread enough that only the NSA could realistically break it with Sybil flooding. But Cryptonote can't scale to widespread. Would I2P become widespread enough on its own?
I have no idea how many i2p users there are. I certainly don't expect Monero to drive i2p adoption to a large degree. Monero is just one small application that will be using i2p.
and what % of internet connections are compatible with NAT traversal?
Here is one claim of an 85% success rate:
http://stackoverflow.com/questions/23655243/nat-traversal-probability-of-success-using-stun