if the relay nodes are Sybil attacked, and many people assume they are because who is providing all this relay traffic for free.
There are no dedicated relay nodes in i2p the way there are in Tor. I2p relies on a bit of social engineering for relay nodes, which is that relaying is turned on by default. So if you are using i2p, you are a relay node by default, and it can reasonably be assumed that most never change defaults. Even if a few do, the rest provide a large relay network sort of for free, but sort of in exchange for the benefit they receive by using the system.
You are not a decentralized relay if your internet provider is blocking STUN
NAT traversal tunneling, which many do I think to stop you from using your internet connection as a server. ISPs have an incentive to force us more and more towards an asymmetric client-server model and away from client-to-client form of P2P.
In any case, just because every legitimate user could potentially be a relay node (but probably isn't because of NAT traversal failure), doesn't mean the relay nodes can't be Sybil flooded (attacked). Remember the same servers running Tor could apply their bandwidth to I2P and present the I2P network with as many IP addresses as necessary, i.e. one relay node per IP address.
Tor showed that some are willing to donate that free bandwidth without any return (unless the return is monitoring all the traffic). And that I2P obscures who is providing free bandwidth, doesn't make that willingness go away. Worse yet, I2P obscures that an entity is Sybil attacking.
Edit: 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? Doesn't currently seem like it, and what % of internet connections are compatible with NAT traversal?
Edit#2: I have serious doubt that I2P can't be denial-of-service destroyed. My theoretical understandind is it is impossible to allow decentralized Sybil attacks as they do and be immune to DDoS. I read a formal statement of this fundamental tradeoff between decentralized Chaum mix-nets and DDoS in some research paper, but I can't remember which one it is. And working through the analysis in my mind, it seems to be the case.