Let $M$ be a holonomic $D_n$ module, where $D_n=\mathbb{C}[x_1,\partial_1,\dots,x_n,\partial_n]$, so we work in affine space. What is the easiest way to prove that any endomorphism of $M$ has a minimal polynomial? By easy I mean some method that does not use the Riemann-Hilbert correspondence\whitney stratifications and such. Preferably it would be a purely algebraic proof. Can this somehow be deduced from the existence of Bernstein-sato polynomials?