The comparison is incomplete, you say your optimizing, but not in the melee example? Using multi-classing to prove the deficiencies of a pure class without addressing the benefits of being single class is silly. Also your comparison only takes into account a power spike in character level at 11th but not bothering to look 12th or higher. If you want to smite more grab full caster or paladin levels after 14th. But for the sake of just the math. I'll assume 12th level, 20 char, with the proper feats and invocations.

Elven Accuracy and 20 char.

Round 1 - Bonus Action Hex Curse - Action Eldritch Blast (3d10 + 15 char + 12 prof) Avg dam 43.5 dam
Round 2 - Bonus Action Hex - Action EB (3d10 +3d6 + 27) avg 54 dam
Round 3 - Bonus Action Quicken EB - Action EB - 108 dam is they all hit

Total: 205.5 dam or 68.6 dam /per round

Hexblade:

GWM and 20 char

Round 1 - Bonus Action Hex Curse - Action 2 attacks + smite 4d6 + 5d8 + 20 char + 8 prof - 64.5 dam (+20 dam- 84.5 GWM)
Round 2 - Bonus Action Hex - Action 2 attacks + smite 4d6 + 5d8 + 2d6 + 20 char + 8 prof - 71.5 dam (+20 dam - 91.5 GWM)
Round 3 - Action 2 attacks + smite 4d6 + 2d6 + 20 char + 8 prof = 49 dam (+20 dam - 69 GWM)

Total: 185 or 61.6 avg/ round (245 dam or 81.6 dam/round ) and this is without haste (item or party buff) or magical weapon.