[yummyfajitas]

HFT has clearly lowered spreads. This isn't even in dispute.

The issue of market makers pulling out during a crisis is a regulatory issue; an market maker might do the right thing and push the market back towards where it should be and then be punished by a regulator who breaks the buy trade. If this behavior is undesirable then eliminate the "clearly erroneous trade" rules.

Your citation also doesn't address any specific mechanism by which a delay would improve things. All it does is speculate that speed might be bad due to fat tails and handwaving.

[msellout]

If we hypothesize that speed might be bad, shouldn't we do an experiment to decrease speeds?

You also haven't addressed the point that many exchanges currently implement delays in times of crisis.