Sorry that your marketing efforts didn't work out. My own experience is the opposite. Hundreds of thousands in sales through the App Store thanks to successful marketing plus the huge quantity of traffic the store offers. Far more than any single software seller is seeing on their own site alone.
Have you not been to an Apple Store? They still have software on the shelves. It is still a thing.
You may be selling just fine without the App Store. That's awesome. Go you. Keep it up. What I've seen in my own experience is that while we sell several million in software a year through out own site, the addition of the App Store means ADDITIONAL sales. More on top of what we'd see selling on our own site alone. Seems you're willing to turn away that additional revenue stream and that's fine. But many of us are seeing huge benefits from diversifying out distribution and finding additional sources of sales.
You're looking at the cut Apple takes from a funny angle. With modern software distribution it costs you nothing to give away a copy of your software. If you sell 1 copy or 100 or even 10,000 copies, the cost to you as a developer (producing that product) is the same (yes there is some additional support cost and a few others but the price is practically the same. Because of this, if you can sell additional copies through the App Store which you wouldn't have on your own site, even that reduced profit is all profit. If you get $70 instead of $100 per copy you sell through them, that's still $70 more than you would have had without them and it costs you nothing additional to sell that extra copy of your software.
It sounds like you're happy with your sales and making a stand to not use the App Store is important to you. That's great. There are however thousands of other developers out there happy to give Apple a small cut because it means sales they never would have had without being listed in the App Store and it makes them far more money.
There are thousands of developers who like Apples 30% cut (I don't call that small btw, that's almost 1/3rd revenue before business taxes even) because they don't even need a website or handle payments. They don't need content delivery networks if their app is quite large and so on.
But there are also many developers who are not on the Mac app store. Spotify is not on there, neither is Photoshop or Illustrator (by Adobe). These companies don't want to give Apple 30% if they don't have to.
Yet both companies are on the iOS App store with products. Why? Because they have to be. Spotify hates giving Apple 30% of subscriptions which is why they tell people don't signup through the app, do it on our website instead.
Again 30% when you're making millions and millions of dollars like you claim to be might be a drop in the ocean but for other people things aren't so great.
We trialled on the Mac App Store for over a year and saw very little movement. Just being on there doesn't help anything. But at the same time we sold the app on our website for the same price and saw a lot more conversions because when people post about us on forums they link to .. our website. Not the Mac App Store. When people talk about us on social media they link to .. our website and not the Mac App Store.
Another thing to consider is that we do not only sell our software on the Mac. We also sell Windows versions. So it's difficult to spend money to send people to only the Mac App Store when we could instead send them directly to our own homepage that offers both client software etc
Am I saying the Mac App Store makes no sense for anyone? Of course not. If you're in that top 10 list you'll be making bank, if Apple features you, you'll be making bank. But there are literally hundreds of thousands of apps on there many with no reviews, no downloads and no revenue being produced.
So again it comes as no surprise to me that Panic would not release Transmit 5 on the Mac App Store. It does not work for everyone.