I'm trying to understand where the market maker = necessarily HFT comes from. After all, we had markets makers well before HFT too. I understand that MM can be particularly profitable when doing HFT, but why is HFT a necessity?
In real terms, if you quote a penny wide market, there's a much greater change that you can both buy and sell in a short period of time to capture that penny. However, if you quote a 10c wide market (when everyone else is quoting a penny), you might buy shares at $10, but it might be a much longer time before anyone wants to buy them back from you at $10.10 - in fact it might be never, they could go straight to 0!
Again, quoting that 10c spread is perfectly valid, it just means that your edge begins to be less about capturing "flow" as much as it is about predicting the direction of the stock over a longer time horizon.
The best way to keep risk low as a market maker is to keep inventory low, which means you need to get out of positions quickly, which means you need to have competitive prices to increase your chance of interacting with order flow.
That's all true for the very strictest definition of a market maker, which is pretty much necessarily HFT (in list US equities). But there are other types of traders that are successful of course, because their edge comes from thinking about the market in a different way. In situations where finely-tuned, data-informed, low-latency algorithms do not have the confidence to give good prices (e.g. illiquid securities or during times of volatility), these traders might be more successful.