I'd presume that someone who is better versed in wall-street style algorithmic trading could speculate better than i what a mature cryptocurrency trading market would look like.
I'm curious to see what a mature market in this space looks like. It'll surely be different on a currency-by-currency basis, because volume, market depth, and fundamentals matter greatly and are unique on a per-currency basis. Time will tell.
As to whether profitable automatic currency trading competes with mining for compute time, I posit the following question to you: Are the two mutually exclusive? In a world with (1) scalable compute power via AWS, Digital Ocean, etc, and (2) both are profitable, why not do both instead of one?
This isn't hypothetical, the CPU mined coins aren't close to profitable with web services. And the people making profitable ASICs are using them rather than selling them because it's intrinsically more profitable.