Not an issue, already solved in many market: offering the normal pricing doesn't stop the provider (here Apple) to offer other special terms to those who want them.

Example in another market: you can buy an Unreal Engine license, or you can get it for pretty much free but then owe a % of sales.

Nobody is saying Apple cannot offer an alternative "30% all included flat price", the complaint is about them offering ONLY that.

The issue is that if Apple lose a tonne of money when Spotify decides to go flat fee, someone has to make up that difference. So smaller Devs lose out as they're currently being subsidised...

Free apps maybe, but then again the "all free" is a myth since there are costs to pay by someone, and Apple is free to keep subsidizing them if they so desire (because an entirely free app is good for their customers).

But non free apps with a smaller base you would be surprised I guess. Storage for hosting and GB for bandwidth is incredibly cheap.

To give a frame of reference AWS pricing is currently at $0.023 per GB for storage and $0.09 per GB for bandwith out.

That means 30% of a $0.50 sub is enough for covering the entire storage + distribution costs of one gigabyte for that user (and that's assuming you need to store an entire copy for each user, which you don't). Yes, there are other costs (website, review, ...), but then again how many subs are $0.50 ?

But what if you're selling content that you aren't making more than 30% profit on? There's flaws for both flat and percent. Something more nuanced might be required.

What if my revenue won't pay the rent?

My point here is that the percentage is based off the price you're selling at, not the amount you're making. So someone who makes a profit of $0.50 off a $1 sale keeps more after Apple's cut than someone who makes a $0.50 profit off a $3 sale.