I think it depends on what you mean by 'balanced.' This takes as a given that the AC a lightly armored, DEX maxed PC should be roughly comparable (within a point or so) to a PC that invests in heavy armor along with a more modest investment in STR. Taken in isolation, only looking at the AC calculations as described in the OP, that can appear to be the case. But the benefits of boosting Dex (as well as the benefits of boosting Str) go far beyond the AC boost gained. The Light Armor is likely getting an attack and damage boost to go a long with an AC boost for every DEX increase, whereas the STR character is only getting an attack/damage boost beyond 15 STR. DEX also keeps benefiting things like Initiative and a more common save (and thus more valued by the design of 5e). Skills may be considered a more nebulous comparison, as campaigns tend to vary a bit in what skills come up more often, but things like Stealth, Acrobatics, etc. are also enhanced by DEX increases, while STR is usually limited to Athletics and the less used (IME) lifting and carrying capacities.