I completely agree that liquidity is first and foremost, and that in today's market structure HFT drives a lot of it. Retail enjoys a lot of the benefit, because trades in the $1,000s - $100,000s range probably aren't enough to slip the market. But is ultra-low-latency the ONLY way to bring about that liquidity? I have yet to come across any economic or technical reason why that has to be the case.

Also, slowing down trading doesn't mean eliminating HFT, it's a matter of what constitutes "high" frequency... I'd argue we're well past the point where incremental increases in speed result in equal gains in liquidity. And those increments now cost more than ever before.