Why is it a "straw man"? Doctors and McDonald's workers are paid differently in a market economy because the supply of McDonald's workers is far greater than the supply of doctors. If you don't agree that income should be determined by the market, why should doctors make more?
It's a straw man specifically because no one is arguing for this. A straw man argument is where you take a version of your interlocutor's argument that is similar, but in some way absurd for critically flawed. Then you attack that critical flaw as a proxy for tearing down the entire argument.
Here's the crux of it: "A world where doctors and McDonald’s workers are compensated exactly the same is one where something major is obviously broken."
Yes, it is obviously broken. The implication here is that the person arguing that "income inequality is bad" is unable to see this completely obvious thing, and pointing it out wins the argument. Because the poster has now "shown" the other position to be obviously wrong and broken.
Except it started with a misrepresentation of the position in order to arrive at that obviously wrong argument. Therefore, strawman.
> If you don't agree that income should be determined by the market, why should doctors make more?
Point of fact, doctors salaries are not set just by the market, because there is an artificial limit on the supply of doctors. If the market truly set doctors' salaries, they might be a lot lower. So saying "income inequality is bad" is not the same as saying "market dynamics are bad"; because here, as in many sectors of our economy, a perversion of market dynamics have caused gross income inequality.
How can you be so sure? That's what it literally means. The onus is on them to better understand and convey their own position, if that is in fact their position.
Most people mean that the gini coefficient is too high, not that it should be zero. Income/wealth inequality is a floating point scale somewhere in the range of 0..1 and neither extreme is likely to be possible or optimum.
But that is a fucking mouthful which is why normal humans just say things like 'wealth inequality is bad' -- until the internet's favorite pedantic muppets show up in the comments demanding clarification.