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 users decide on the software he runs fully aware of the rules it enforces.
The point is miners cannot go and start mining invalid blocks on a different chain and expect the economic majority behind Bitcoin will follow, they won't.