DEXes are born after CEXes. When DEXes are completed and matured, CEXes take a majority of market share already. One more reason is that, people are too familiar with centralization and need time to get used to with decentralization. In my opinion, DEXes are the ultimate destinations of crypto. Sooner or later, Binance DEX or IDEX, Fork Delta...will replace Binance, Okex or Kucoin.