Current density depends not only on metal line length (= poly width), but also on the current flow pattern in multi-layer metallization, that is determined by the topology of the layout.
For example, if a metal line is used to route the current vertically (in Z direction), then the lateral current density in the metal is nearly zero, and metal line length is not a factor for EM reliability.
Also, even if debiasing is no uniform (over poly width, or over device area) - but matched between the two devices - then this non-uniform debiasing will not impact device matching.
Sheet resistances of metals do increase, in general, with technology scaling (especially strongly starting from ~16nm node).