it is not about quantity of decentralized exchanges it is about their quality. so far 90% of the projects that are calling themselves "decentralized exchange" are created only to sell people a certain token they also create and raise funds to get rich because of it. and these exchanges don't have that much volume to begin with not to mention that they usually only support a limited number of coins mostly only the ethereum tokens not even altcoins!
so i don't think any kind of big transition from centralized to decentralized exchanges happening any time soon.