This is not true. One person cannot fork the code. If one person creates a fork of Bitcoin that is not accepted by the other miners and the network then the newly forked code will be worthless as will the coins that any miner mines using the unaccepted code.
Bitcoin is decentralized because no one entity controls the network. In order for anything to be changed, there will need to be an agreement with the majority of the network to even have the possibility that the changes will be accepted.
Obviously, the value of Bitcoin to a single user alone on a forked blockchain is very small.
The value of any bitcoin fork is equal to the combined value all participants place upon the fork. The result would typically mean that the value and security of the forked "original" bitcoin version would be greatly diminished and some changes to the algorithm would have to be implemented to prevent attacks from the miners that didn't come along. There are many solutions to this as have been discussed and tested.
Your statement is provably untrue because all the PoW alts represent forks of Bitcoin which started with very small userbases and grew.
In fact, it is trivial to fork Bitcoin as you don't even need to be a developer as some companies will even help you do so for 28 bucks.
You are correct to say that altcoins that use PoW are essentially forks of Bitcoin. You should also notice that the value of the vast majority of altcoins have decreased over time, especially as of late.
You are incorrect to say that the value of a Bitcoin fork is the value of the combined value of the bitcoin of the participants of the fork. If the market does not give any value to this bitcoin then it will not have any value, and the bitcoin on the unforked network will still retain it's value.
This is why it is risky to mine when the network is about to hard-fork as if a miner does not start to mine on the new fork (and the new fork is accepted) then the miner will be mining a worthless coin that will never have any demand and will be on a network that has zero security/ability to have transactions get confirmed.