From a purely technical viewpoint the consensus is based on mining hashrate as mentioned in the post above.
its not mining hashrate
for instance imagine you owned a pool with DOUBLE the hashrate of the entire network. but your codebase of your pool node wanted to change a rule..
all that will happen is that your blocks will get rejected by the network because their desired rules are not your rules.
you could mine billions of blocks but only you would retain them and everyone else would reject them. thus you have made your own altcoin that only you hold
consensus is about the NODES that accept the rules. hash power has nothing to do with the RULES. the hashpower is only about protecting the DATA from people manipulating the data retrospectively. EG for someone to make a block that does not contain certain transactions (attempted refund) from a block in 2012. then that pool needs to also recreate every block after that to get uptodate to todays blockheight. and the rules must match to not get rejected.
in short hashrate does not determine the rules. it just makes changing old data harder
Excellent point, nodes are also a very important part of the Bitcoin ecosystem, which is why keeping a lean blockchain is important for decentralization. In practice there's nothing stopping miners from mining on a fork with poor node density though, as propagation will still find a way one way or another. It's just that network stability is likely to suffer, in addition to increased centralization towards the network segments with the best access to full nodes.
the politics of CORE controlling the rules.. is the thing people are avoiding talking about. because its core that set the rules and push people to download their wallet as oppose to other wallets. thus they control the development and they control the rules. and yes they do and have bribed big names to get their way. (many free all inclusive weekend breaks, jobs, bonuses, freebies and more)
The market has decided in favor of Bitcoin Core's design philosophy and development approach over that of its hardfork competitors. Anyone who believes that BTC's market support is only caused by market manipulation and astroturfing should rejoice over the possibility on getting their hands on cheap hardfork coins until the market finally decides to wake up.