> A substantial amount of trades in the market are executed by liquidity providers that provide efficient two-sided markets. These important trading firms trade on both sides of the market and are largely risk flat at the end of the day. Real-time settlement would require these liquidity providers to have significant sums of capital and securities on hand to make trade-for-trade deliveries. The current infrastructure relies on the efficiencies of trade netting to accommodate substantial amounts of trades. Wholesale market-making is important to the infrastructure as it provides the capital and balance sheets to back customer trading. Significantly increasing those capital requirements by requiring pre-funding of all trades without the benefit of recognizing the offsetting trades would substantially increase the costs of trade execution to the end user. Real-time settlement could also introduce operational inefficiencies. In a typical low-volatility trading day, NSCC and DTC process over one million shares per second on average, valued at over $16 million. During peak trading hours, such as market open and close, this number spikes to over 300 million shares per second, valued at over $25 billion. Settling these amounts throughout the day in real-time would introduce substantial financial and operational risks to the equity markets. Accelerated settlement that abandons the significant capital and operational efficiencies gained through centralized multilateral netting would be a step backward for the world’s most liquid markets. Striking a balance between the capital efficiencies of netting and the risk mitigation benefits of moving settlement closer to trade date should be the goal of the industry.

Thanks! Replacing market makers seems difficult.

Edit: Why the downvotes? I'm quoting the article for easy access.