If you are the manufacturer, who already know the root private key and can make some fake users, you still won't be able to print new money from nowhere. Yes, you CAN make double spending, but this will just split the network, because every participant will accept only one spend, not both.
Who and how issues the new money? how do you decentralize that? Otherwise you have a central bank and a database is more efficient.
That’s a problem that has already been solved in various ways by existing cryptocurrencies. For example, NANO was premined and then distributed via a faucet that required solving CAPTCHAs. Other cryptos just award holders more coins, or use other solutions.