Yeah I thought r/bitcoin was a good place to argue against the idea that any altcoin is automatically subsumable by bitcoin. Needless say the downvotes came pretty quick. I think the chance that Bitcoin simply changes to match any competitors is ludicrous. Only consensus changes are possible whilst preserving the Blockchain. Any controversial change that has enough merit will split the market between Bitcoin and an altcoin and divide supporters along those lines. Meaning the holders of Bitcoin become more resistant to change as the altcoin becomes more successful. Its definitely not clear that scenario ends in a change to Bitcoin early enough to defeat the altcoin.

I also think the OPs point about the highlander effect isn't well made - a coins mining pool only needs to be strong enough to protect itself from attacks that it's value incentivises.