Large volume of transactions still goes to centralized exchanges but of course we love to have a decentralized wxchange where there is no KYC and limiting in transaction that if you have a large or volume of transactions to be made. But, whatever it would mean too risky for we do not know that platform can be a phishing or just waiting for the right time to run away with bitcoins. Being careful is a must may it be a decentralized or centralized.
Once DEXs can handle heavy liquidity and they're guaranteed to be safe, the last vestige of the CEX imo would be liability insurance if they have it - i.e. CEX gets hacked, loses BTC, insurance pays back some of the money. At that rate, though, CEXs would be more vulnerable than DEXs anyway. However, you cannot expect much from DEX, because DEX only makes transactions easier so it is impossible to expect DEX services to be better than CEX.
I don't quite follow your logic here 