Yes, but even homo Economicus isn't trying to maximize expected income, they're trying to maximize expected utility. Unless you're completely indifferent to risk, the sure thing is better.
That’s not even snark, humans preferring both of higher risk and lower payoff is a canonical psycho-economic result. And even the question of whether the concept of a rational utility maximizer is well-founded is the exact subject of this very fine article!
Sure. I very much doubt anyone seeks risk because it's risk. They do it for the thrill, or for social reasons, or because calculating risk is hard, etc.
> having any other computable utility function on any other measurable that we’d have to accept as equally valid if we want to convince any non-economist that we aren’t just using utility as a weasely word for money
Almost anything else will do just as well. If you can buy it (or the means to acquire it) on the market, then there's always some point at which twice as much money buys less than twice as much. Maybe 10 million is not enough to hit that point, or 5 million is too much, but the curve flattens out somewhere. And if you can't, then the money makes no direct difference, but can still buy you food and shelter and free time with which to pursue your other projects.
I suppose in principle you could actively want to not have money (and not want to give it away either), in which case getting rid of 10 million is probably not twice as hard as getting rid of 5, but that hardly seems realistic.