If I sell 10,000 copies of a $5 game on Steam, Valve takes a 30% cut.
If I sell 10,000,000 copies of a $5 game on Steam, Valve takes a lower cut of the sales past a certain point. I don't know the exact numbers but it's true, I read it on the internet.
If you're a struggling game developer Valve takes a higher percentage of your revenue, making it harder to be a not-so-struggling game developer.
Valve has a lot of money and could be more generous to indie developers who aren't selling many copies.
Anyone who read this line and didn't realise this is a troll is an idiot.
The problem with your position that they should be more lenient to small sales games than big ones is:
A: It punishes success, and their biggest customers. The current system rewards success.
B: It promotes sending out low-quality games that won't sell well enough to soar past the threshold
C: It's not reflective of the actual reason why costs drop like that (which is because many services are actually easier to manage in bulk than in small scale, even digital ones)
Hell, I'll just use your examples.
If you sell 10,000 copies of a $5 game, that's $50,000 = 30% steam cut for 15000
If you sell 10,000,000 copies of a $5 game, that's 50,000,000 = 20% steam cut for 10,000,000
The big seller is still paying several orders of magnitude more than the little one.
Also, if your indie game is selling less than 10k units and you can't make a profit, that might be because you're a shit developer. You're either investing way too heavily in stuff that's overexpensive on a very niche game such as professional voice acting or multiple writers, or the game's just... bad. Good indie games can easily pass hundreds of thousands and get at least to the first threshold.