Because if they're not using the same node software as the exchanges, they can't trade with other people trading the BTC instrument. The exchanges allocated the BTC ticker to "whatever the bitcoin core node software says it is" and that is defined by the one guy who has commit access to the bitcoin core repo.
Wrong, because even if the exchanges pick a chain that is completely ridiculous in terms of rules, say they permanently limit the tx throughput for the entire world to 3, or something absurd like this, and the exchanges and users who can see how absurd this is sensibly opt out of mining or transacting on that chain, the profit for defecting is great, because mining is a zero sum game.
As long as some suckers are trading actually liquid assets for the instruments on the sabotaged chain, it is economically rational for the miners to mindlessly rubberstamp the rules of the chain.
Former Bitcoin Cash developer Amaury decided he wanted a subsidy to himself. He forked, forming his own extremely minority coin, Bitcoin ABC (now eCash) which pays him 8% of everything mined. Despite being very conclusively rejected, and the chain having undergone attacks, eCash still retains some value, and Amaury still gets to pocket a percentage of everything that gets mined.
The underlying code for that is the same as for BTC. So at any time, if the cabal thought cratering BTC would be worthwhile if they got to pocket enough of what remained, they could do it with complete impunity. All it'd take is the agreement of a very few people and a viable exit plan.
That's an interesting thing I didn't realize until not very long ago. If setting fire to 99% of the ecosystem allows you to pocket a fraction of the 1% that remains, and that works out well enough to not have to work again -- that's a very favorable tradeoff for a lot of people.
Right, this and BTC both, although Amaury's proposal was less insultingly stupid than the proposal that split BTC (I require an 8% of the coinbase donation to continue optimising this chain vs I require this chain be completely dysfunctional and useless, and force the intermediaries back into it for it to be of any utility at all, when the entire original purpose of the ledger was to disintermediate them)
