Centralized exchanges are needed for trading, decentralized exchanges are needed for swap, where you use your wallet. It's safe and fast, but expensive. Centralized services are prone to scam and hot wallet attacks, decentralized exchanges are prone to vulnerabilities in smart contracts and rug pull scams in liquidity pools. I can't say any platform is better than the other as they both have their advantages and disadvantages. There is no perfect solution.
Actually, I think exchanges are venturing into swapping. Like the one I use Bitget has a swap for different tokens and also has a cold wallet. I don't really know much about other centralised exchange because I haven't researched deeply to know if other exchange supports it as well.
most of the exchange also offer swapping like you said like mexc and kucoin literally has a way to convert instantly without the need to make orders.
but of course they will take fee and even sending money to the exchange itself is already charging you fee so basically its the same thing.
the different as mentioned is that using cex, if one is trading their assets frequently, they could save a ton, if only ethereum blockchain or any blockchain thats popular like bitcoin are cheaper i think dex could become popular
but right now, people prefer to save money from fee, since its not some small amounts either.
i guess both would still thrive though for upcoming decades.