In a flat space things are simpler. For a standard Cartesian coördinate system the metric tensor is gij = δij which is Kronecker’s delta. An expression like xjkgjk in the context of a curved manifold becomes xjkδjk in a flat space with conventional Cartesian coördinates. In turn xjkδjk = xkk but at the cost of abandoning the rule of upper and lower scripts. The upper-lower rule is not needed when gij = δij.
The upper and lower rule ensured that only vectors from a space and its dual were “multiplied”. The distinction is unnecessary in Cartesian coördinates.