The bottom line is that phones are commoditized at this point. There's nothing (statistically nothing, that is -- obviously everyone has preferences) that people want them to do that they don't, and there's nothing that a premium product can offer that the cheap ones don't have too.
That's not something Apple or Stratechery or posters here can fix. It's just the way technology works. It's the same thing (almost exactly the same thing) that happened to Apple in the early 90's. Sure, macs were still better. But they were better and more expensive and for 95% of what the median purchaser (which at the time meant "desk worker at a US business") wanted to do a PC clone running windows was perfectly fine. So Apple got trapped in a slowly shrinking prestige niche.
Now, eventually they broke out with exciting new products and new markets that drive new success. But that's not a "business decision".
This article uses all the data that is publicly available and yet claims that somehow Apple executives missed all that.
Stratechery isn't some random site, but you're comparing it to a guy on twitter.
A precise estimate isn't necessary. His claim was that they would shrink measureably in the first S year after a redesign.
Edit: The guy I follow on twitter is not some random dude. He is an analyst and makes some great points.
But now that things are getting more different, such as prices becoming much higher, a possible economic downturn around the corner, mid-range phones becoming "good enough" for most people, the loss of subsidies from carriers, and so on, Tim Cook is getting a little "lost" in all of this, and doesn't know the best way forward. Worse, he may have already chosen a way forward (Apple becoming more "fashion-focus" for instance) and it may be the wrong way.
As a high end product, the mac is likely not gonna make them much more profits than it already does. However, additional market share would further strengthen their customer base for the areas which will make them money. iPhones, iPads, and services.
If Apple were to release a 5-600$ laptop it likely wouldn't make them much money, but it would create more mac users, and I suspect mac users are significantly more likely to own iPhones and pay for services like iCloud and Apple Music than Windows and Linux users.
And TBH, Apple has a massive market in the enterprise just waiting for some competition. You can see this in the rapid and popular uptake of Google @ Work services, and Chrome OS in the enterprise and education services. People were looking for a simpler alternative to the entire MS Exchange ecosystem, and once again, while these may not have been massive money spinners on their own (though they definitely could have been that as well), they would form a strong solid base to sell iPhones to.
This would kill Apple. It's a profit share company. It's a profit share culture. Customer and culture are tightly knit; you don't change one without altering the other.