I’m sorry but it’s not even that. It’s the user experience that matters. I don’t care if the UI is optimised for mobile or laptop, big or small, when the user experience is bad. It is to me incomprehensible how so many mainstream, otherwise successful, applications allow themselves to give the user a bad experience still today.
One example for all: connectivity. Think to how many apps need to be online to work, even to provide the most trivial of features, such as a “like” in Facebook. I live in London, mobile data is great but, anyway, I’ll find myself on trains and in tunnels, switching from the surface to the underground, with most apps becoming “incapacitated” while offline, or switching network from 4G to WiFi etc. whatever my device. That’s a 20th century problem that we’re pretending not seeing. Can’t Facebook and Twitter really “capture” my like-ish stuff and upload them later? How hard is it for Spotify and iTunes to pre-emptively cache the music I’m listening? Why can’t my train journey planner store automatically the timetable of the commute I’ve been doing every day since 2005?
We should learn putting ourselves in our users’ shoes well before starting speculating on what the size of their screen is.