[Lucid:]

Given your example, what if the enemy ADC has 2 kills? Now with your two kills you were able to buy a bunch of wards and an emblem of valor or a kindelgem. Your carry got 2 assists and has boots and a vamp scepter. But the enemy ADC has a bloodthirster. That is not beneficial to your lane at all.

Yes, a support getting a kill will always be better than nobody getting the kill. But the ADC will always be able to make better use of that gold. Only First Blood or killing somebody on a killing spree is worth 400 gold, and assist gold is only half the kill gold split between assisters. If jungle ganks and support gets the kill, your carry only gets 100g for first blood or 75 for any other kill. If the adc can get the kill, that's about three full waves of minions they don't have to farm before they get a damage item. If the support gets the kill and the carry gets an assist, then the carry is only about three measly minions closer to their damage item. And that's assuming that you're a diamond-level player and can get every creep.

Supports are designed to be able to run on little to no gold. If you watch the pro games in the LCS, many of the pro supports buy a sightstone, boots, and more wards. They don't buy Zeke's, they usually let the jungler buy Aegis/Locket, and the support is valuable because of the utility inherent in their kit. If Lulu, Janna, or Nami weren't allowed to buy any items, they would all still be perfectly functional supports.