That's true, but where owning a substantial amount of Bitcoin comes into play is once a fork has happened, those who are large holders have an outsized influence on the price of each fork.
That said, it is also risky to sell off all you own of one fork when it may not end up being the one that comes out on top. Only those with a lot of conviction about which fork SHOULD be the winner will do this, and there is more conviction among those who aren't just trying to get rich.
I don't believe dumping has real power. It certainly has influence over short term oriented people but not those who believe Bitcoin SHOULD be over one or an other.
I actually think this might be the real question. How do we know which fork *should* be the real one?
That's based on each user's conviction about what Bitcoin is, how it should scale to serve as global medium of exchange, and whether they even believe that is a goal that should be aimed for.
Should also mention that each user has to determine what tradeoffs are worth achieving their goal.
For instance, the blocksize war was not between one side that wanted Bitcoin to be able to scale to allow everyone in the world to buy coffee and another side who didn't want that at all. Rather, it was between those who wanted to try and achieve this by making it impossible for the average person to run a full node, and those who wanted to scale in layers so that as many people as possible could still run a full node for the sake of maximum decentralization and protection of the protocol from malicious changes.
