The desktop PC became the dominant computing platform almost 40 years ago with the introduction of the first generation of affordable home computers (Apple II being the primary example). Even though laptop sales have overtook desktop sales around 10 years ago, the preferred form factor (15"+ screen) and intended use (sits at a desk 90%+ of the time) of the vast majority of laptop owners means the majority of laptops are nothing but desktops that happen to be easier to move to another desk.
Even though the amount of time spent using desktops and laptops in a desktop-like manner is decreasing, it's still a form factor found in every home. There may be more smartphones in that home but in my opinion, any product which is universally present in people's home is dominant even if it isn't being used as often as other options. Another example of this would be the microwave versus the oven. I'm not sure which one is used more often than the other but both are still dominant in terms of their universal presence in every home.
Even though the amount of time spent using desktops and laptops in a desktop-like manner is decreasing, it's still a form factor found in every home. There may be more smartphones in that home but in my opinion, any product which is universally present in people's home is dominant even if it isn't being used as often as other options. Another example of this would be the microwave versus the oven. I'm not sure which one is used more often than the other but both are still dominant in terms of their universal presence in every home.