Joy-Con joysticks use a potentiometer to read the voltage at a wiper that slides across a strip of resistive material. That material wears down over time, or plastic and dust can dirty the sensors.
Stick drift is a huge problem with other Switch models. One survey found that 40% of Switch owners had problems with their Joy-Cons drifting, and things didn’t get any better with the Lite or OLED editions. After a bunch of lawsuits, Nintendo’s president even admitted it and apologized, setting up a free repair program for customers in some parts of the world.
While I agree with the overall message, I'd say this is on Nintendo for not doing proper communication on the topic. They must know this is a major concern for most people who had a Switch.
Yes, I get they don't want to acknowledge the drift problem publicly, but surely someone on their marketing team can still come up with a way to discuss the improvements they made and alleviate fears.