As I previously stated, the miners are the only entity that cannot fake their level of support. It is trivial to run an arbitrary number of fakes nodes,
The point you're trying to make is a valid one, but your expression of it isn't technically correct. Miners can easily fake their 'support' for any flag, and they often have in the past-- e.g. signaling BTC1 when they weren't running it and had zero intention of ever running it.
I suspect major mining pools are running their own custom implementations of bitcoin for broadcasting, receiving, and validating blocks.
In 2017, there were many "futures" markets that allowed market participants to price the value of various fork-coins, and the owners of mining equipment ("hashers") likely saw that mining on the NYA/BTC1 would not be in their best interest.
So while it's true that miner signaling is sybil resistant, the mining pools signaling flags don't necessarily have much skin in what they signal, so fairly little incentive to do so honestly. This is particularly true because the parties with an investment in hardware are hashers (whom don't control the flags) and not pools (who do control the flags.
The mining pools earn income from the blocks they mine, and they need to invest in infrastructure to allow their pool to operate. If a pool has fewer hashers, it will have less income. It is also trivial for hashers to move their miners to pools that are signaling for or against a proposal that is in line with the long-term best interest of bitcoin. If a pool is signaling for or against a proposal that is not in bitcoins best interest, it will lose customers.
Many pools have significant altcoin investments and it's not too hard to imagine a kickback or altcoin play making fake signaling in a pool's interest. Even hashers can move hashpower to altcoins that might gain value if Bitcoin had issues.
I would have to disagree with the
bolded part of your statement. The altcoins that can be mined using ASIC miners that can also mine bitcoin have a very low network hashrate as a function of the hashrate of the bitcoin network. If 1% of the hashers (measured by hashrate) currently mining bitcoin were to move to bitcoin cash, the difficulty would more than double, which means the price of bitcoin cash would need to more than double for the hashers to have the same amount of revenue. This is simply not something that will scale for enough hashers to oppose a bitcoin proposal whose failure would benefit bcash. Even if it did scale, once these hashers start mining bcash, any proposal whose failure would benefit bcash could subsequently be implemented.
The real unfakable metric is the eventual market price of the asset -- but we can't know that in advance, only make educated guesses.
I mentioned futures markets for various fork-coins above. Any futures market would need to either be centralized, or run on an altcoin blockchain, such as etherum in order to be reliable.
My position continues to be that it is
best to get miners' consent prior to implementing a fork (soft or hard). If there is a BIP proposal that is receiving pushback from the miners (based on what they are signaling), or if the miners are signaling support for a "bad" BIP proposal, a good next step would be to encourage reputable exchanges to open futures markets for both "for" and "against" forks for a BIP, and hope that the result will change minds.