My (grand)parents like their FOSS launcher, gallery, and chat client just fine. I've had zero questions about how Signal works, but a bunch about how to deal with the OS' pre-installed garbage spawning notifications about this or that update. They can't tell the difference between an advertisement pushed by some commercial app they want and a smartwatch firmware update notification

From my POV, it's the commercial software that has fundamental usability issues due to misaligned incentives (not completely different either, but not as aligned as FOSS). They just have a better lobby and marketing budget. Chrome didn't become this ubiquitous on mobile by having to be downloaded from f-droid, but by making a deal that device manufacturers cannot refuse