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.
Valid is subjective of course, and the changes to that are decided by the dev team. Very perceptive! That's how it's always been, seeing as even now Bitcoin is still a work in progress.
The system is already decentralised in the ways that Satoshi imagined. Yours and the "Bitcoin Unlimited" crowd's mentality involves letting the user loose with deciding every aspect of the system. It's lunacy, it's the opposite of what's been going on up to now, and it would end with all the different "Bitcoins" disintegrating, apart from the overbloated centralised one perhaps
Valid being a subjective term is indeed a good observation. The question still stands of who decides? Obviously you think that the Core developers should decide for us, I think that the decision making process for Bitcoin should become more distributed instead, the future of Bitcoin should be decided by the economic majority which as far as I understand it is also how Bitcoin ultimately still functions today.