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You lack the the economic knowledge to understand that crypto has to be turned into fiat in our day and age ?
Thats where trusting something like coinbase becomes a non option.
--snip--
I don't have economic degree. But i'm aware of these option,
1. You could store all Bitcoin on self-custodial wallet.
2. You could just send necessary amount of Bitcoin you wish convert to fiat.
3. Depending on where you live or fiat currency you use, you could avoid CEX (centralized exchange). Other reader may want to visit
https://kycnot.me/ to see list of P2P (peer to peer), DEX (decentralized exchange) and no/optional-KYC exchange.
4. There are some places which accept Bitcoin and some other cryptocurrency as payment options. Other reader could search "who accepts bitcoin" or "list of place that accept bitcoin" to find such places.
5. Idiom "Don't Put All Your Eggs in One Basket". In this case, it means don't put all of your asset/money in one places (Coinbase).