It's true that having a single stack makes things more efficient, but it also makes the process less adaptive. Although Apple seems to have a policy of buying out/copying successful software and integrating them into the OS, they can't buy everything. So eventually the more open/diverse ecosystem is going to have the better value proposition - a computer is a general-purpose device, and the less it can do the less useful it is. Of course this assumes Apple never gets market dominance where it surpasses all other companies combined, but phone sales are maybe 20% of the market and the laptop sales are only 8% of the market, so this doesn't seem likely to happen.
tl;dr 80 developers working on an open stack, even with coordination friction, are going to outperform 20 developers on a closed stack.
Your tl;dr assumes that the 80 developers are going to coordinate at all as opposed to silo themselves with duplicative efforts (e.g. GCC, Clang, MSVC; Intel Core, AMD Ryzen; Windows, Linux, BSD).
It also assumes that more developers is inherently better. For a rebuttal, see The Mythical Man Month.
I see duplication as a good thing. For every app on the App Store there are 1.5 Android apps. These extra apps mostly likely do similar things, but they offer extra features or perhaps are implemented more efficiently. In comparison the Apple apps will not offer these extra features/performance, following a one-size-fits-all approach. The more diverse ecosystem wins, because people have never really prioritized eye candy over usefulness. Of course, economically Apple development is more profitable, the same way that COBOL development is more profitable than webdev, but this is only a short term effect.
You're thinking about it wrong. It's not 1 app on iOS for every 1.5 apps on Android. For any given app in a competitive space, there are probably twenty choices on iOS and thirty choices on Android.
It's fair to say that three choices is better than two, but every additional choice will have diminishing returns. And at some point, the chances of an additional choice having any novel appraoach or distinctive features approaches zero.
tl;dr 80 developers working on an open stack, even with coordination friction, are going to outperform 20 developers on a closed stack.