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.
Yeah, I've always believed that myself about the mobile market. The people who spend their time on free to play games aren't the same as the consumers in the dedicated handheld space. Anecdotally, I used to have a co-worker who would play Candy Crush on his work PC, but would rib me for talking about actual video games on consoles and handhelds (and we were the same age).
I don't doubt that there was a Covid boost in early 2020, I just have my doubts that the Switch's sustained sales throughout the year are owed solely to that. And as I mentioned, I find it odd that devices that center around the living room and internet access got less of a boost throughout the year during a period of lockdowns and social distancing, than a piece of hardware which features the ability to leave the home.