You can't even fork a stablecoin. in case of a split in a PoS chain, the correct fork will be decided for you by USDC and Coinbase.
Unless we're going to pretend that there is only one way on and off networks and only in one currency denomination…
In practice the value of the forked collateral is likely to be low, leaving the stablecoins insolvent.
However, if you are only forking the vm and allowing for people de deploy other protocols (or forks of other protocols), this is not the case (they just start off at lower total supply relative to the native collateral available on that network from a lower demand base).
I think id agree for things like ZCash/Dash etc compared to BTC, but I'm not sure I'd agree when it comes to the all contracts deployed on all EVM networks and none of this has anything to do with decentralized stablecoins.
For example, you can mint MIM (a decentralized stablecoin) on both avalanche c-chain and ethereum (as well as polygon, fantom, bsc and arbitrum), and they are both worth $1, but have different collateral backing it on both networks. If users wanted to leave one or the other, they could just redeem their mim for the underlying, sell it and buy the collateral on another network and mint it on the other network. The collateral might trade lower on one network based on market factors (like if the narrative shifted to that the chain became too centralized or w/e, and this assumes that even the price movement of the underlying overwhelms the over collateralization ratio, it might not) but it would just mean that there would be more or less mim on that particular network as assets are liquidated and not that the MIM itself would be worth less.
If you want to use coinbase to buy crypto and tokens, that's on you.
It gets a lot more complicated than that because setting up competing systems is cheap. It is like saying "nobody would write this piece of software for free". What we learned with open source is if the cost of distribution gets low enough then there needs to be just one person somewhere on earth willing to maintain it and it can work.
If the cost of creating trustworthy local (or international) monetary systems is basically 0 then it isn't obvious that plutocrats have an advantage beyond the one they already have by virtue of being powerful. If they can force you to use their system they already control the government so didn't need any technical help.