It is possible to combine the advantages of dex and cex but it will be costly to create this kind of hybrid exchange due to its complexity. However, the more complex structure a platform has the more expensive it will be to maintain security. There are already some popular exchanges out there so it will be hard to make a position in the market. The most challenging part will be getting enough funding. You need to assure investors that they will be profitable within a few years of service from the exchange. The idea isn't bad but the probability of creating such a thing is challenging.
In short, it's clear that you're emphasizing that CeX and DEX can't really be the same, something that I also agree with and have some truth to. But I also can't say that my Cex and Dex are useless because they also help me at other times.
Although I've been in this industry for a few years, when it comes to exchanging crypto with our fiat currency, I use CEX to make it happen via p2p features that Cex has, like Binance, to our local exchange in the country where I'm located. Because I'm not sure if there is p2p that can be used on the Dex platform, is there?