Isn't Jane Street mostly into HFT arbitrage?

There's that, but the market in the West was already over-saturated with HFT arbitrage. What's hot is growing markets that get a little less attention. The big Jane Street maneuver in India made a lot of noise recently. They've been "banned" for "market manipulation", but that was one of their biggest play to date.

You need market makers like Jane Street to have well functioning markets.

The markets have shown themselves to be an excellent way of applying human potential to things like crop harvesting efficiency, new medicines.