the reason for your question is because the centralization exchange (cex) first appeared before the Decentralization Exchange (dex), if if first the DEX appeared first I am sure many people would prefer DEX over CEX now,
if DEX is already many and famous then I'm sure many people will switch to DEX later.
Your reason isn't entirely correct. You see, Regardless of which happened first, People would often opt for things which are easier to access and a couple of other factors. Dex has been existing since late 2016 if I remember correctly and majority of them have fell short. I think about 2 of them now requires customers to do KYC in order to increase trading/withdrawal limits. One other thing that makes dex far more valuable is because they have little to no volume even Binance dex is struggling despite most of its volume coming from Binance.com . Decentralized Exchanges have a long way to go
I agree, IDEX volume went dowhhill after they hade implemented the KYC in the platform. I also stopped using it since its somewhat the same with a CEX exchange.
Also regulations will be a big issue for operating a DEX exchange especially the FATF thats against money laundering.