The handheld market was never really threatened by mobile. Sure, the 3DS didn't do as well as the DS (a hard feat to replicate under any circumstance) and the Vita flopped, but that could just as easily be due to factors specific to those systems rather than competition from without. But I never really believed it was because people were ditching handhelds to go play Candy Crush and Angry Birds. A portable console is just that: a device that offers console-type gaming experiences on a device that's not tied to a television. The types of games on offer and popular on handheld consoles are typically far different from the types of games that are popular on phones and tablets. To liken it to personal transportation (I'm not the best with analogies, BTW), consoles are like automobiles, handhelds are like motorcycles, and mobile is like bicycles. A motorcycle may look more like a bicycle in terms of form factor, but as a motor vehicle it's closer to an automobile in terms of the role it fills.
Also, the COVID bump is a real thing because the data clearly shows an across-the-board increase in demand for consoles, not just the Switch. Just putting that out there since you referenced it. The Switch would still have had a strong 2020 without COVID, just not as big as the actual 2020, maybe another ~20M year instead of the 25-26M it actually sold (according to Nintendo's own reported sell-through figures).