I could do that myself now: make a copy of core, change the PoW formula, and presto: a hard fork happened, and the coin has split. If the 1000 PH/s is not important, what is the difference between my bitcoin and their bitcoin?
For a start, the PoW.
After that, it depends.
Depends on what? If the hashpower is not important, and preserving the PoW is not important, then what is the magic factor that would cause all the value to go to the CoreCoin branch instead of the CartelCoin branch?
Whatever it is that the economic majority thinks is more important. If it is sticking with the existing miner fleet, that's where the value ends up. If it is firing the existing miner fleet, that's where the value ends up. There is no answer you or I can state without that information that is more correct than another.
ELI5 the mechanism for the the "economic majority" to make its wishes known & relevant?
The market.
Assuming I hold 90% of all the coins mined to date (I'm economic majority), how do I go about this shit?
If you own 90% of the coins, you are the market. Obviously not realistic. In reality there are many participants, and they'll trade with one another and work it out. One fork will be worth more than the other, probably a lot more, but who knows.
90% was chosen at random. Let's say I own 5%, and keep in in cold storage, as a "store of value." How do I make my voice count?
Keep in mind, you edited out the important part:
<>If I'm just holding the coin (not solding), why do my wishes matter to the miners (I do not buy, only hodl)? To the exchanges (I do not trade, only hodl)?
According to your reasoning, by not trading (hodling the coin in cold storage), I effectively make myself irrelevant. Correct?
Hell no, holding is what gives it value.
If you simply hold both, the split of value between the forks does not matter to you at all.
Assuming anyone else values this Bitcoin shit at all of course. If no one does, then you're screwed, same as if we wake up tomorrow and no wants Bitcoin any more.
If you want guaranteed store of value,
independent of what other people value, too fucking bad. That doesn't exist. About the best you can do is buy a chocolate bar, at least you can eat it.