No sneak attacks shouldn't be a toggle. You are allowed only one sneak attack per turn. And when your rogue will get more than one attack a turn, you will be happy to choose on which of them you can apply your sneak attack. This is why it's a skill and not a default thing.
Let me give an example. You are level 6+ and behind an enemy who has 1 HP left. You won't waste your sneak attack on him if you can just kill him, move behind another adversary and use your sneak attack on a target where it will definitely be more useful. Add even more attacks per turn as you level up and you get the picture.
That's wrong, If you miss mainhand attack sneak can be applied to offhand, it's something Very importante actually to keep rogue viable, as having only one chance do deal significant damage at high level makes one of the lowest dps on 5e just straight up the worst by far.
Talking about your example, that's the reason for a toggle, you disable it, kill the enemy, reenable for next attack, it works for both cases, currently it works only for the case where you never miss your main attack
