I do think that Bitcoin is a peer to peer network, ultimately it is the peers/nodes/people that decide what Bitcoin becomes. Since it does reflect the will of its participants, if that will becomes split then so does Bitcoin. I do not think that a 51% split is always justified but it certainly is possible both theoretically and technically.

If the network is forked by the majority of the mining power and it is not supported by the people then that fork will not succeed because people will choose not to use it. This is why miners are incentivized not to go against the economic majority or the will of its participants, because it would be impossible to do so over the long run. This makes the interrelationship between the full nodes and the miners more complex. It is the miners that initiate the fork, while it is the nodes that ultimately decide which version will become the most dominant, this however is part of a process that does begin with the miners.

I use the term fifty one percent mainly for theoretical reasons since a split would be much more likely to occur when the majority introduces a fork which a smaller minority then refuses to follow. Which I think is within their right to do, in the same sense it is within the right of the majority to initiate a fork through the miner majority. It could be that over time the weaker chain could grow to become the more dominant chain again. If it proves itself to be better and more people choose to use that chain instead. In this way the market decides and people decide. This is the root of freedom behind cryptocurrency. However this does mean that the eccentricities and abstract nature of humanity will be reflected onto cryptocurrency as well with all of its flaws and beauty.

Whether a split will happen depends on the conditions. I think it is only justified when there are fundamental ideological disagreements that can not be resolved. I think it is highly unlikely that this will happen with this blocksize debate, however I am rather certain that we will see this happen in the future, most likely over entirely different issues. It is a radical concept but it does lie at the heart of the Bitcoin protocol and it will help secure the freedom presently inherent in Bitcoin. Even if the majority does not choose freedom, we will always still have the choice. This is the technological gift of financial freedom that cryptocurrency has given us.