I'm not saying Apple don't provide a service, of course they do, though at times it sucks. My main point is about is about the price, not the principle. I pay Apple a yearly subscription just to develop for their platform. On top of that every sale I make they get 30%.
Would 95% be ok?
Another extreme element of this is that I cannot sell a licence outside of the App Store to my customers. I cannot ask them to buy from my website (which many have asked about and would be happy to do) and provide them with a code they can use to unlock or upgrade. This is explicitly banned. Unless you're a big player like Spotify or Netflix your app will not be allowed to do this. I've tried and been denied and stonewalled.
In this case I am willing to build and manage my own payment/licencing setup to gain 30%. I'm just not allowed to do that.
And I don't, tbh, have an opinion about the rest of your post; I'm talking about Steam, not Apple.
But, having gone down this road myself (and why I cite the Jeff Vogel talk): you would almost certainly spend more than 30% on customer support and payment processing and fraud protection and accounting. Or you would do it incorrectly, which is worse. And, regardless, you'd expose yourself to a way smaller audience. It seems to me to be a good way to shoot yourself in the foot for no gain (and as an app developer myself I certainly wouldn't bother).
Steam's bigger marketplace and that it removes the uncertainty and risks around payment provider management (chargebacks etc.) easily make up for the bigger cut. Do less work, make more money. It's not unreasonable.