> team can probably afford to rewrite most of their software periodically

I think Google partially does this in order to keep its engineers happy, as you are more happy when you develop something from the ground-up compared to just maintaining something already built. By going down this route those engineers are kept in a “happy state” so there’s less risk of them flying off to other pastures, where they could potentially build the next product that could “kill” Google. Sort of invisible golden hand-cuffs, if you will. I personally find it tremendously wasteful at a societal level but I can see the value of this strategy for Google as a company.