You seriously can't tell that Tim Cook is simply pandering to Trump's vanity to try and dissuade him from applying his tariffs to Apple products, which would affect Apple's profitability?
Here's what (I believe) is happening on the ground.
Phones are quickly becoming good enough, resulting in more people opting to hold on to their phones for longer periods of time. Despite fewer iPhones being sold every year, the iPhone installed base continues to grow via the 2nd-hand market. This is possible through a number of channels, such as Apple's annual upgrade program, as well as their trade-in program which helps to offset the price of a newer iPhone. This is helped by iPhones generally retaining their resale value better, the long-term software support that iOS devices enjoy, and Apple's ability to properly refurbish 2nd-hand iPhones before reselling them in the grey market.
What this also means is that if the newest iPhone on the market isn't compelling enough, existing iPhone users will likely simply elect to hold on to their iPhones longer. Sure, there will be those who eventually choose to switch to android phones, but not in any meaningful quantity that would cause Apple execs sleepless nights. While there is no doubt alternatives to existing apple services on the android side, it may not be so easily to transition all over at one short, especially if there really isn't any benefit in doing so. So if you tell me that in order to switch from an iPhone to a Note 10, I need to repurchase all my apps again (and some don't even have android equivalents), lose imessage and airdrop, sell my Apple Watch etc, how many will just say "screw it, I am sticking with my iPhone for another year?"
Meanwhile, Apple is transitioning from selling iPhones, to selling to people with iPhones. We see it in their focus on wearables (Apple Watch, rumoured AR headset), more accessories (AirPods, rumoured bluetooth tracker), higher prices and more services. Heck, the Apple Card likely earns Apple a higher cut than their 0.15% commission from Apple Pay. So Apple doesn't need you to keep buying iPhones; they just need you to keep using them.
When you want to analyse iPhone sales, you need to do so within the context of what Apple is doing as a company in totality, otherwise you will miss the big picture every time.
In a way, I kinda feel sorry for Samsung. They are being squeezed from both ends. They are losing out to cheaper android handsets from Chinese OEMs, and cannot loosen the tight grip that Apple has on the more premium end of the market. But such is the fate of one who has the temerity to call themselves an iPhone killer, I suppose.