This is why CEX is still cheaper because you are paying % trading fee but that is usually few cents and not too much, maybe if you are dealing with a ton of money you could be leaving a huge chunk on the table but for lower end of the spectrum we are talking about just few cents. Which is why I think it is highly important that DEX finds a way to drop the costs, or ETH needs to find a way to drop it, one way or another transaction fees are killing DEX's right now.
The fee costs of either a centralized or decentralized exchange are relative. Uniswap has high fees because the underlying blockchain network it's built on, has been facing high transaction load. The limited capacity of Ethereum, causes fees to skyrocket in a blink of an eye. It's no wonder why many decentralized exchanges running on the Ethereum blockchain are so expensive to use these days. Fortunately, there are solutions available to tackle the "high fee" issue. Second-layer projects like OMG Network and Zk-Sync aim to reduce fee costs on the blockchain. Decentralized exchanges using a second-layer solution could experience a major reduction in fees, making them as good as their centralized counterparts. Even the upcoming ETH 2.0 scalability upgrade will have a positive effect over fee costs on the Blockchain. Centralized exchanges may have lower fees than their decentralized counterparts, but they're often subject to hacks and theft. It's why decentralized exchanges (especially those using atomic swaps technology) are the best choice for trading crypto-to-crypto in the safest way possible.
Nonetheless, atomic swaps are certainly the way of the future. But the technology is just starting to blossom. Given that centralized exchanges have long dominated the crypto space since its inception, it's unlikely they'll go away anytime soon. At least, people will have a choice to trade crypto without the middleman in a seamless manner. Atomic swaps are better than ordinary decentralized exchanges because they're completely non-custodial (no need to host an exchange on a centralized server, etc.). Imagine being able to exchange BTC to LTC in a quick and easy way directly on the Blockchain without interacting with a central server. It'll certainly increase crypto's adoption all the way to the moon. Just my thoughts