Of course there are limits to every opportunity. I make money everyday from opportunities that exist on microsecond timescales. I run arb and loan liquidation bots, so I haven't had a single losing day, ever. The last two weeks were quite profitable for me, actually the best two week period I have ever had. There are other strategies that are easier to pull off, such as market making, that have moderate risks and outsized returns.

The point is that these opportunities exist, and they always will exist, you just have to be smart enough to be able to get into them. If someone needed money for infrastructure to run an arb bot, for example, and offered you above market, risk-free returns, it's at least possible they aren't lying to you. That was my point.