There are several points here...
Windows rots, after a while you need to re-install it. If anything a better machine rots faster because it runs faster.
Windows acquires an amazing amount of crap as it runs, and eventually the sheer weight of this brings your machine to a grinding halt.
Last time I saw a manufacturers numbers the life expectancy of a laptop used as such was 18 months. But a lot these days are used as small desktop machines, so average life is 3-4 years.