I continue to not be surprised by these sales numbers. From the moment I saw the reveal trailer, it held the immediate appeal of a 'cool hybrid tech toy'. I was in freshman year uni and hearing the random buzz and excitement about this device was refreshing.
The past five years, I've felt 'Switch' has grown to join the litany of phones, smartwatches, tablets, eReaders etc. as this personal multi-use device. Similar to how Game Boy was inspired by the Walkman success (down to the name) and combined previous innovations like interchangeable carts, d-pad, and low-powered LCD display to create a new brand and device type, Switch has done similar. It's not just 'handheld with TV out', it's an entire UX effort.
The OLED upgrade slots right in with this - just as folks upgrade their phones with bigger / better screens and hand down their older devices, it becomes viable for someone to trade in their Switch or give it to someone else in favor of an upgrade.
Since Switch is not a singular unit of hardware but a brand and a 'category' of gaming device, they can pull a Ship of Theseus and replace literally every component while still retaining the Switch aspect. They have already begun this with the OLED - replacing the kickstand, chassis, speakers, and screen entirely, retaining compatibility with joy-con. A "Nintendo Switch" in the year 2030 will be a 'Switch' on the surface but it could have an entirely new screen, body, joy-con, and OS. That's pretty exciting to think about.