Good job on reaching this realization - many people never get there. You have the nail on the head: What is the point of using a decentralized currency if you leave it entirely in the control of centralized third parties? Indeed, using bitcoin via centralized exchanges is not just as bad as using fiat, but in many ways it is actively worse. CEXs lock accounts for secret and arbitrary reasons for more often than fiat banks. With fiat banks, there is a legal process you can go through to have your account unlocked, whereas with CEXs you usually just lose everything. CEXs exit scam and disappear at an alarming rate. Most banks are insured, while most CEXs are not.

If I hold cash, then yeah, it it going to constantly devalue, but the cash will still be there tomorrow. If I hold bitcoin on a CEX, then who knows if I will still have access to it tomorrow, next week, next month?