Developers are not competing with Apple, they compete with other app developers. This is what is going to prevent them from raising the prices. Without 40+% Apple tax the developers can keep the same prices (before Apple tax) and maintain their profits. The consumers will benefit.
It's not the point I am trying to make.
Say I am a developer who made a game that is currently selling for $10 in the App Store. Going with basic economic theory, a higher price would mean fewer downloads, while too low a price may not net me much money despite having more purchases. I would have arrived at this price because I will determine that is what will generate me the most amount of revenue, and which is a fair representation of the value and work that went into crafting the game.
Whether Apple takes 30% or 15% or even 0% will have no impact on my final pricing because once it is done, software on the App Store has zero marginal or distribution costs. It's all pure profit at this point. The only thing that matters is to maximise revenue.
Say Apple allows a third party App Store on iOS in the future and I have the option of selling the same game on both the iOS App Store and the Epic game store. Even if the Epic game store collects just 12% from me, I would still charge $10, and keep the difference for myself. I wouldn't lower the price because again, I would (based on some guesstimates) not earn enough from the increased sales to offset the lower profits.
What this means from the customer's perspective is that the price of my app or IAP is not going to change regardless of which App Store it can be found on. There is no money to be saved for them, and as such, there is no upside to having multiple app stores on iOS at all (with regards to price at least).
That's why I believe Epic overplayed their hand back last year. No support came from their use base, not least because consumers don’t care about a 30% fee that they never see, and surprise surprise, people don’t actually dislike closed, sandboxed app ecosystems (which is why Epic eventually brought Fortnite to Google Play after initially launching it as a standalone launcher).
TL;DR - the benefits you and a few others love to tout here online probably will not resonate much with Apple's user base in general. Not least because it will not be giving them more of what they want.