If your ADC can't cs properly

Well, minor differences in CS won't decide any game in any way really. It's when large disparities come in that games start swinging. When large disparities come in, it's usually one of two reasons.

1. The other guy is winning lane and pressuring you off CS
2. You're being too aggressive and foregoing CS for trading. (I am hard guilty of this)

If you're bad at last hitting, that's not something an item should exist to fix, but rather just some time spent in either norms or the practice tool to memorize the health threshold of melee, ranged and cannon minions at various stages in the laning phase. Sounds daunting, but it's just how it has to be done. The only way we could justify a last hit item would be to have it have some serious drawbacks come later, since while that may fix the problem with people missing some CS, what were to happen if the person beating the lane and CSing great has the item? It would be just a current version of that champion with more gold, you know?

Winning lane isn't just coming out of it with more kills and less deaths than your opponent, or taking the tower first. Creep control is a large part of it, and part of winning lane is forcing your enemy to take less CS. However, this imbalance only heavily exists in the laning phase since once its over, the winning champion typically has more gold and power, so they're supposed to run around the map and push their advantage/gold lead elsewhere. This gives the loser time to quietly farm and catch up, which is what you're supposed to do losing. So, with proper wards, the winner runs around and gets nothing done while you are CSing the entire time. Doesn't ever work perfectly like this, but it's how it's generally supposed to work, not taking into account individual player decision making relative to their ELO.

It's hard to tweak the difficulty of CS, because you can never buff only one ELO. If you make CSing easier in some way, you'll help out many bronze players sure, but then you go up to Diamond and above and you'll see that whatever power you gave players in Bronze with the extra bit of gold they earned, you'll be giving high elo players exponentially more gold since they'll last hit much better with buffs than bronze. This in turn will cause early game dominance, and a meta will revolve around that bringing back all the champions who try to make a match 20 minutes long. Sounds extreme, but when 1-4AD buffs and 5 MS nerfs cause outrage, you can believe any way of increasing gold at every stage of the game will be problematic.

Practice makes perfect is all that it is. If you CS poorly, you're going to have a rougher game. If you were perfect at CS, well then you should be high, high ELO. Otherwise, it's completely natural to miss CS and its all part of the lane win conditions. If you have someone on your team way behind in CS, it will hurt you, same as the person who died 5 times in 8 minutes. The goal of the lane is to accumulate your gold and hinder theirs. You could tweak CSing somehow, but all you'll do is shift the power budget up. So say we attach some arbitrary numbers in regards to CS performance in lane, say the winner is at a value of 6 while loser is at 3. Tweak csing, and the winner will be at 8 while the loser at 5. While yes one could argue the loser received more gold percentage wise, it won't cover the lack of CS and it will just push someone's advantage further.