You have to realize, it's not "your gold, teammate A's gold, etc."
It's your team's gold vs enemy team's gold.
It doesn't matter who gets the killshot, it's beneficial to all 5 of your team members.
Even when you're talking about carry and support, it doesn't matter if the support happens to get the kill.
The carry still gets the assist, and the real gold advantage for them as an individual is from what's done after.
When a kill happens, the killer gets that gold, assisters get assist gold, then with the enemy dead people get cs gold, tower gold, and jungle objectives.
In the case of you doing all the work alone then someone coming up and last hitting, that is 100% better than you just getting the kill alone, because then your team gets kill gold and assist gold, instead of just one.
Even without the gold matter, the choice at hand for your ally is not "let you get the kill" vs "take the kill and give you assist".
The choice is "take the kill and give you assist" vs "the enemy doesn't get killed".
Which means your team doesn't even get the kill gold at all, forget the assist.
In more direct words, kill stealing doesn't exist.