Yes, exactly. If the market is perfectly competitive, and cost to switching is low, margins trend towards 0.
The same should be true in software, regardless of how cheap it is to produce an additional unit. Of course, because it's cheap for anybody else to produce that additional unit too.
The reason we see such high margins in software is a combination of "first mover advantage", pseudo-monopolistic like elements to the business, and the fact that industry is so new, VC money does not tend to go towards creating direct competitors as there is ample opportunity elsewhere.
In the short run, first movers and innovators will be able to generate high margins of course. And sometimes business X really does produce a better product than the rest, as some would say Apple does. But given that software is cheap to maintain once built, you would expect many viable and roughly equivalent competitors to be built in the long run.
At the end of the day, legislation should be written to foster "competitive capitalism", as that produces the best result for the public at large.
For example, it should be as easy to switch between SaaS providers as the flick of a button. Apple should have to allow competing app stores and not give preferential treatment to their own... And so on.
If you think about the App store example in isolation, the fee they charge is a direct transfer of wealth from app creators and app buyers to Apple. Would they be able to charge 30% if there were competing app stores? Maybe, but I doubt it over the long run. Certainly at least they wouldn't be able to charge Epic that amount, as Fortnite is big enough to entice people through side channels for loading the app.
Looking at it as choice of Apple or Android is not an appropriate level of granularity for anti-trust IMO.
We must recognize that markets created within a platform also must be competitive, not just the gateway into that ecosystem. Cost of switching becomes high, which leads to de facto Monopoly power by the platform owner.