But what classes as excessive?

That's a good question, one that I have not defined for myself perfectly.

I think part of it is the nature of the transaction. When you sell something off your Etsy shop, you create a thing, you sell the thing, you can't sell the thing again. A shop like Steam continuously takes money from you for the exact same service. Of course it takes money to run the servers and any other running costs, and I'm not saying those shouldn't be covered. But theoretically, if they have set their automated systems well, Steam runs by itself without intervention from anyone. Whoever owns Steam basically makes money on their sleep. They created it once and it continually makes money for them.

When a game sells well, this game will be downloaded more often, so the relative load/usage of the Steam servers increases. So it is fair to take more money from games that sell better, so tying it to "amount of games sold" makes sense. But does the load on the Steam servers really change if a game is sold for 50€ or 10€? No, what really matters is the size of the game, the amount of updates the developers push and so on. So tying the costs to sale price is also not necessarily fair.

Apart from that, it's hard to define something as "excessive" without comparing it to other things. As I mentioned once, I don't think a teacher is doing a less valuable job than a CEO of some big company. Most jobs are benefitting others/society in some way, so I actually value most jobs roughly the same. In conclusion, I would define as "excessive" anything that is a large deviation from mean income, completely arbitrarily I might say if your income is more than double the mean, it would be excessive.

All profit is excessive by nature, isn’t it?

I don't necessarily think so. People die, so their accumulated wealth disappears or is transferred to someone else. Human beings are made to acquire more resources. But death is a natural endpoint to this process. There is probably an equilibrium point of profit that is sustainable with a certain population.