PC sales decline has very little to do with people moving to other platforms or form factors and is more a result of PC's now being powerful enough that the lifecycle is much much longer. 10 years ago people were upgrading every 1-2 years, before that it was sometimes every 9-12 months just to try and keep up. Nowadays a 5 year old PC is good enough for just about everything and even for gaming you can survive on an old rig or spend a little just to upgrade the graphics. This makes annual sales decrease significantly but doesn't really reduce user base.