In the short term, there are two clear "classes" of exchanges: custodial exchanges and non-custodial exchanges. Today, the latter category is just DEXes such as Uniswap, and in the future we may also see cryptographically "constrained" CEXes where user funds are held in something like a validium smart contract. We may also see half-custodial exchanges where we trust them with fiat but not cryptocurrency. Both types of exchanges will continue to exist, and the easiest backwards-compatible way ...