Since 1990, US population has increased 30% while the (inflation corrected) GDP has increased 90%. So something improved. I'm going to (arbitrarily) say it was computer literacy, since home PC ownership went from 15% of households to 85% of households in that time period.
What can you counter with to say that the improvements were from market efficiency? If we never had HFT, how much lower would the GDP be?
It sounds like you saying that private companies that don't participate in the stock market are incapable of managing capital or achieving growth.
401k is a great indicator, but I'm not sure it makes HFT look very good. The median trade time is more than 1000x faster than in 1990. But people's 401ks are not doing 1000x better. Did we hit a point of diminishing returns long ago? If so, is HFT pointless?
I feel like you are conflating HFT with electronic trading. Electronic trading can be HFT or slow, either way it is cheap and cuts the middleman out.
You seem to have this backwards notion that they need to justify their existence, they don't, they're just traders executing their rights to buy and sell like everyone else. It's those like you seeking to regulate HFT that need to justify yourselves. You don't even understand what HFT is really as you're asking basic questions like what does market efficiency mean and does HFT help it, and you think you're in a position to question someone else's trading habits? Really? HFT doesn't need to look good, those of you trying to punish them need to show some actual evidence they're doing something bad, but they're not and you can't.
Efficiency in a market means things are priced accurately and you're not getting ripped off when you buy or sell; if you find that vague and hand wavy, well, sorry but that's what it means and you should be able to understand that without further explanation. HFT traders make the price more accurate benefiting you and everyone else, they don't need to justify themselves, you need to justify your witch hunt against them.
HFT is a form of electronic trading, I'm not conflating them, they're just different forms of using tech to trade and there's valid reason at all to single either of them out as bad. HFT are market makers, they're providing you and everyone else liquidity for a vastly smaller fee than you've been provided it ever before. So say thank you to HFT, enjoy your cheaper trades and increased liquidity, and go find a real problem to complain about instead of attacking those who make your life better.