We have seen this 'longest chain' verbiage where 'valid' is notably excluded for some time. I take it as a backup or auxiliary attack strategy. Every opportunity is taken to lay the foundation participant misunderstanding however. That misinformation effort seems to have picked up again over this last month here on bitcointalk.org.
Basically, if a bloatblock (for instance) can be inserted just one time, it would fork the chain and obsolete Bitcoin core software.
So what if a given version of bitcoin software is obsoleted by evolution of the network? The "core" developers have done this themselves several times. Arguments along the line of "longest valid chain" do not resolve the issue under debate. They just move the debate to the meaning of the word "valid".
Despite some advertising claims, the security of bitcoin does not depend on mathematics alone, it depends upon running software, which intern depends on running hardware which is supported by the economic majority. Any other way of defining "valid" necessarily requires the argument from authority, which ultimately results in a debate over who is the authority. With this, any hope of a decentralized system goes out the door.