[Dopedangel]

I think the more valid question would be how long will Windows and Microsoft last as most other companies are slowly taking over areas where it dominates. Yes it is still dominates but in areas that are slowly loosing relevancy.
Apple is hardware company despite making huge profits it still has not reached it growth peak. And every 2-3 years it is entering new markets so I don't expect it to die out anytime soon. 10 years from now I expect it to be making self driving cars.
Where as google is concerned although it dominats search and advertising it also has not reached its growth peak more than 70% of the world population still does not use the internet for financial transactions. And as the smartphone penetration and mobile internet speeds increase so will Google grow.