However, the Half-Orc's major racial trait currently only benefits melee classes. Half-orcs will always benefit more from being a Barbarian than being a Sorcerer or a Ranger.
Umm... Ranger is a melee class. You can literally play it as full 2 handed melee. Nothing precludes you from doing so.
Two handed melee classes will still benefit the most from this racial if it applies to all attacks, so I can't see it being a balance issue. Barbarians can swing a battleaxe multiple times per turn for extra D12's of damage on a crit, whereas most cantrips are D8 or D10 and they rarely get more than one cast per turn.
Keep in mind that "All attacks" includes more than just weapons and cantrips.
It will also affect unarmed attacks - So Monks spamming a dozen d8 punches a turn.
Divine Smite will benefit making Half-Orc Paladins significantly stronger.
Not to mention spells too, where you have multi-hit spells like Scorching Ray and Magic Missile which while not the biggest dice, their multi-attack rolls increases the chance of crits and benefits from the passive.
With of course AoE spells, especially high damage ones like Call Lightning, Ice Storm, Flame Strike and Destructive Wave which while they only hit once per turn, can hit multiple targets with individual rolls each and provide significant damage increases with some of their dice (For example, Ice Storm would get an extra D8 + D6, Flame Strike/Destructive Wave both get 2 additional D8's)
It might not be the biggest of deals, but it's worth keeping in mind that such a suggestion isn't just "Well at best you get Eldritch Blast for 3 D10's!"
So while I think it could be considered opening up Savage Attacks to affect more things, the effect it could have would need to be looked into to ensure it doesn't simply go from Orc = Barb/Fighter into Orc = Some other class that gets mad benefit from the extra dice on crits.