What do you think the benefits to you would be to pay someone else for the exact same software.
I understand that you may want just to have Apple as your supplier. What I don't understand is how DMA denies you that ability. Which seams to be the core of your concern.
Now incurring the risk of trying to answer what seams to be an rhetorical question ...
Competition usually leads to better deals than having a solo supplier that you are fully dependent on. Prices tend to rise has the dependency gets stronger, not the other way around. That is market behavior one-o-one. The economy of scale do not to the customer as there no reason to.
Quite the contrary. Regardless of the economies of scale companies look for ways to capitalize on the dependency.
Coming up with layers and layer of payinf feature walls for trivial technical things in order to get more money. To the point that things that were free suddenly you are paying for dearly as part of the a compound. We used to have one flagship iPhone for 600… now we have how many up to 1.5K? A glorified file distribution system that takes 15/30% slice of the price of sale of any software or digital service.
On another note your properties are more protected when the supplier acts against your interests considering your properties.
Just the other day Apple banned Epic apps. A situation that has nothing to do with you as an iPhone owner. Say you love to play Fortnite on your iPhone. It does not matter who fault was, in the end it had a negative impact on your property. Just the other day Apple announced no PWA support in the EU after the DMA requirements, again this situation is will hurt your properties.
In a deal the more dependent you are on the other party the worst you get. Who tells you otherwise is simply lying. You simply get less for the money.
But I suspect you know all this.
Its seams that the DMA is about Apple. It is not. It's about the interface of very large companies properties that the world is becoming ever more dependent on with the rest of the economy.