The Density of Numbers Represented by Diagonal Forms of Large Degree

Let $s \geq 3$ be a fixed positive integer and $a_1,\dots,a_s \in \mathbb{Z}$ be arbitrary. We show that, on average over $k$, the density of numbers represented by the degree $k$ diagonal form $$a_1 x_1^k + \cdots + a_s x_s^k$$ decays rapidly with respect to $k$.