Nothing. Just like nothing stops anyone from printing out their own paper money. However, nobody will accept it, since the trust in your new paper money is next to nil, as no shop or services will accept it as a form of payment (perhaps except yourself).
Replacing paper money with a new cryptocoin of your own makes no changes to the above argument.
However, if you are able to produce goods or services which is needed by somebody, then you can price your goods/services in your own paper money, and setup an exchange rate yourself. If other people sees the exchange rate, and deem it worthy, they will exchange their existing currency with yours, in order to purchase your goods/services. This process is the one that gives any currency any real value.
Is there EVER a non-privacy, government, or tax avoidance type reason to hold bitcoin? I’m thinking like gold / precious metals are used in electronics and industry, jewelry, etc. A coin can’t be held and used for something. I suppose a company can use a blockchain internally for verifying things / ledger / whatever magic use they dream up, but that wouldn’t really be using it as a currency.
I actually am bullish on crypto, overall, because I think people are not rational and will buy for many years into the hype. On a 20-30 year timeline, I think it is harder to predict and know what to expect. Lots of things change when you look more than a few years out.
Someone please convince me, in a concise fashion!