The number of household computers are decreasing? I wonder where you heard that one. PCs are still and will perhaps always be used for the simple fact that its capabilities in terms of media including watching movies, playing games, creating graphics, gigantic disk space, and large monitors are timeless and only improve with the years. Mobile phones and tablets still cannot compare to them, and it is doubtful that they ever will since if powerful hardware can fit in their tight, restrictive spaces, then hardware that is many times more powerful due to the lift of space restrictions will be developed and implemented into laptops and desktops practically forever to fuel these demands.
It's just that more people are using mobile phones in their daily lives away from the PC, that's all. It still serves as an example as to why webmasters should make their themes more responsive to these devices, though, the million of users who use mobile devices is a huge sign that there is a market there.