Perhaps it would be more accurate to say that marginal return is materially positive for all feasible n
In fact, unless the cost of adding and using training data exponentially decreases over time, it's a mathematical certainty that a logarithmically increasing n will quickly incur expensive, diminishing returns for using more data.
So in the context of this Google paper, you could conceive of a situation where training data actually becomes easier to load (albeit subexponentially) and still becomes too expensive to use relatively quickly.