Not all the people prefer decentralisation, as I see. Many of them would like to use centralised services like banks or brokers etc. They don't want to control their accounts and funds by themselves. For example hundreds of people are losing their private keys and doesn't understand that MEW support can't help them in this case 
well, I totally agree with what you say because right now there are a lot of decentralized wallets that have a lot of problems, I won't use that method, it's better if I use another wallet storage method that I think can be safer like a hardware wallet that I can store and control. fully.Its not really that much complicated to understand when it comes to basics.Talking about decentralized wallets? its no different when you do own one which you do able to posses its keys for you to access funds.
It might be nothing usual for some newbies or in short not user friendly but it isnt really that hard to understand.The main thing i do saw why people do prefer centralized exchangers over dex ones is because they
can deal up with fiat tx not like on dex which is purely on crypto to crypto thats why we dont have any choice but to rely on cex ones due to this reason.
This is more reason centralized exchanges will continue to be relevant. Yes, DEXes may find a place of its own relevance, but it is limited.
Then talking about you possessing a private key to a decentralized wallet. What do you think should be said about what happened to FolkDelta before its death. Well, that notwithstanding, irrespective of whatever might have happened to some of the few DEXes, I think the best security still lies there.