Apple pays monthly at 45 days after the end of the month.
So they won't be holding on to money.
It means you'll have a 15% cut taken out of your revenue until you exceed $1.176m (which becomes $1m of your post commission revenue) at which point all future revenue for that year will have a 30% cut (year 1).
You'll continue to pay 30% the following year (year 2).
If at the end of that year, your post commission revenue is less than $1m (ie you earned less than $1.42m pre-commission), then the year after that you'll be back on the 15% rate (year 3).
So there's a gap there that will mean that if you are close to that initial $1.176m near the end of the year, you should stop your sales, unless you expect the following year to have at least an additional ~$300K in sales to make up for the increased commission.
It would be better and more logical to apply the same rule for each calendar year. Start at 15% and ramp to 30% as the equivalent of a marginal tax.
On the other hand, the costs to Apple are essentially flat for all apps, so other than their own revenue, there's no justification for the increased commission.
People can argue about whether developers get value for money from the app store (ie, what commission is appropriate for someone providing billing, local taxation, debt collection, all receivables risk etc).
People can also argue about whether Apple has a monopoly. If they do, it's on their own hardware. So it's not a market monopoly. If you define the market as "app stores for Apple hardware".
People can also argue that Google and Apple have a price fixing arrangement.
They could argue back that they don't, they both independently came to the same commission rates (or that Google followed Apple, so that's not Apple's fault).
They could also argue that they don't control the market, depending on how the market is defined, eg "mobile phone applications" as a market has numerous other app stores.