Simplex-is-the-best conjecture for fixed terminal count

Let $d\geq 1$, and consider finite point configurations of $d+1$ points in Euclidean space. Their Steiner ratio is the ratio of the cost of the optimal Steiner tree to the cost of the minimum spanning tree. Simplex-is-the-best conjecture. The $d+1$ vertices of a $d$-dimensional regular simplex have the minimum Steiner ratio over all point configurations of $d+1$ points in Euclidean space. This is a fixed-terminal analogue of the higher-dimensional Gilbert–Pollak conjecture: the number of terminals is bounded, rather than the ambient dimension. The source presents it as relevant to the computational study of Euclidean Steiner trees; its resolution is not given.