My point being is that the longest valid chain will be decided by the people who are behind the full nodes and miners. Therefore if they choose to support forking Bitcoin in order to increase the blocksize then that will become the longest valid chain, even if you still personally do not consider it valid it does not change the fact that according to the terminology that we are using it is.

The longest valid chain is determined by the miners according to where they point their hashing power. Miners also choose what code they run and therefore what type of blocks they produce. Therefore the miners can fork the network and create the longest valid chain even if the majority of full nodes choose not to follow such a fork. Essentially both the miners and the full nodes have the free choice to run whatever code they want, however it is the miners that determine which side of the fork will have the longest valid chain.