Volume isn't a particularly relevant metric here. Think of it as the resolution HFT firms trade at rather than a measure of the size of their trading activity. It's not a perfect analogy, but the point is that trade volume is not the best metric for understanding the scope and impact of HFT. It's definitely not an indicator that HFT makes up 30-70% of the finance industry in any meaningful capacity.
Here's a thought experiment: how much would things change if HFT firms traded half as much, with double the margin on each trade? Some securities would be a bit less liquid but otherwise almost nothing would change. It certainly wouldn't be a 2x difference from the status quo!
HFT having a lot of volume is irrelevant, what matters for Julia to be prevalent, is to have a lot of analysts use the toolset. You should measure the amount of analysts working in HFT vs those not working in HFT.
Furthermore - HFT is perhaps the worst case for Julia, since the implementation will always be in low level languages. Julia's speedup is only useful for research in HFT.
Maybe not in terms of earnings, but in volume. Estimates range from 30-70%, depending on the market.
I'm digging up references now for an edit...