It has always been the longest valid chain though. If miners were to introduce double-spending transactions in a block, full nodes would--by default--reject the blocks, regardless of how much work the chain contains. This also highlights why miners don't control the network and never had.

I'm no expert so bear with me.

My understanding is that the danger of miner control is not "double-spending" but making multiple spends and being able to pick the most favorable one after the fact.

As a non-controller if you try to double-spend, you can't know which one of your attempted transactions will be recorded in the ledger and which will be rejected. As a 51% controlling miner, you can choose the 'correct' transaction.

This allows the 51% controlling miner to engage in futures arbitrage.

Double-spending just means spending the same output in multiple transactions--there are many ways to perform a double-spend attack. Your interpretation of a 51% attack is correct.

If it wasn't clear, my comment was referring to a double-spend that would result in an invalid block i.e. both transactions are on the same chain. I simply used an example to argue that it's always been the case that invalid chains are rejected by honest nodes.

That's the beauty of it. It does not matter what Blockstream says. The longest chain is Bitcoin.

> The longest chain is Bitcoin.

Useless pedantry that doesn't even map into real world use. The real world has shown your definition is incorrect.

Ethereum forked. Which ethereum fork is the "correct" one? Bitcoin forked. Which one is the correct one? Depends on who you ask...

A sufficiently large miner could, right now, create a block which created a million bitcoins from nowhere and gave those to them and it would - for a while at least - be the longest chain. No other clients would accept it, of course, but by your definition the chain where someone just made themselves a billionaire would be Bitcoin.