Google and apple effectively have a 30% tax on all mobile software business. They are killing all apetite on developing new mobile software, as less and less developers want to be involved with their shady practices of stealing features and locking apps out.
It would be the same if there were only 2 stores where you can buy clothes in the US and they tax every manufacturer 30% to sell any goods there, while also selling their own goods without commission. On top of that, occasionally stealing good concepts, copying those, and selling as their own products. And of course, locking out annoying partners at their will without having to explain why one manufacturer can no longer engage in business with their customers.
This sounds a bit like “nobody drives anymore because the traffic is too bad.” App stores are still growing at a decent clip, no?
You can buy apps from other Android stores as well. Android stores that have absolutely no affiliation with Google.
In fact, even in the grocery retailing industry, there are integrations that would run afoul of the rule you are proposing. Kroger, for instance, sells brands it owns, while selling competing brands for what is, effectively, a commission. I'm not just picking on Kroger, all major chains engage in this practice. I was only using Kroger by way of illustration.
So what you're proposing has far reaching implications. I don't even think it's necessarily "bad", per se. But it does change the entire "order of battle", as it were, in the American retail space. Lots of people would go out of business, and lots more would be in serious financial trouble.