What you are explaining here is the longest chain, without the word valid. While the longest chain is usually valid, it's not necessarily so. Validity is decided by the nodes which are the economic majority. Miners are incentivized to follow the economic majority, but they are not required to do so.
I don't think that's how it works at present. As I understand it, the longest chain is decided by the software, the user has no way to determine which chain the software follows. Might be wrong, but it's a debugging feature if it exists at all.
The longest chain is decided by the software that the majority of miners choose to run. It is the users that determine which chain the software follows because it is the users that choose the code in the form of what type of full nodes they decide to support, the same is true for the miners.
This might be the aspect of the Bitcoin network you are failing to take account of, the human element. This aspect of the Bitcoin network does not fit neatly in the discipline of computer science, a multidisciplinary approach is required in order to better understand the various dynamics that make up the body politic of Bitcoin. From game theory, psychology, politics and economics. Bitcoin might not always be entirely rational considering that it does reflect the will of the economic majority.