With all of the arguments around Segwit vs BU I came up with my own definition for the real Bitcoin chain... It's we the people, who own and buy and use bitcoin, who choose what are the rules we agree to and what chain we are willing and interested in transacting on which gives it it's value, miners just follow that value. Whichever coin has the largest market cap IS Bitcoin, even if its a fork that starts from scratch and burns all the originals money (aka alts). That's why ETH is the real branch even though its a fork and ETC is secondary. Its inevitable that a hard fork will have one illegitimate child that may continue to have some value. If BU were to activate it's success would not be determined by the miners (majority hash power) but by the people willing to spend money to acquire it (miners will just mine whatever is most profitable). I doubt we'll have one coin as a stable store of value used only for large wealth transfers and another coin for the everyday transaction, in this space the coin everyone uses will be the greatest store of value.
The idea of digital gold is that it should retain its value and not have to shapeshift funds around to the coin you want to place your bets in. We should start alts that keep all transactions from the genesis block on and it would make it a lot easier for the existing community to get behind it. If not, eventually there will be new larger population that decides another coin is more valuable to them and its market cap will became larger than bitcoins. At that point it can be looked at as the valid fork of Bitcoin and the new digital gold. Better fork now keeping old balances than letting an alt take over.
Summary: Largest marketcap defines what is "Bitcoin"