HFT makes markets more liquid and efficient. I think people oppose HFT because it prevents small investors from competing with algorithms and bigger players. However, the function of market shouldn't be job creation, it should be used to determine true market prices.
If all traders are replaced by HFT algorithms, that would be fantastic news. Opposition to that is ludditism.
In theory you're right, but in practice, it breaks the mechanisms which ensure that investors actually want to participate in the market.
The function of the market is not job creation, however one of its functions is to incentivise investors to invest rather than store their money under the mattress. HFT hurts that incentive, that's why it's functionally bad.
The purpose of the market is to get all that capital out of the mattresses and into the economy, not to be liquid or efficient. Those are secondary objectives. To get investors' money, it needs first of all to be perceived as fair.
If all traders are replaced by HFT algorithms, that would be fantastic news. Opposition to that is ludditism.