Currently the method highly encourages team play, where as the method you speak of would greatly hinder anyone playing as support and create the extremely irritating complaints of 'team killing' more often than usual. I understand the concern however...imagine you're playing healing sedna or bubbling queen of thorns, you don't deal nearly as much damage but negative defense heals/bubbles. I find it irritating enough that there isn't like overall 'healing' done at the end of the match.
I believe the best suggestion would simply be to split the gold gain to an entire team in general as it is a 'team'.
I.E. buying citadel upgrades does not help you deal damage but can be hugely beneficial to the team....destroying towers/pushing lanes/capturing flags.
Either make it a complete split or keep it as is, I believe that would be far more beneficial to the game and environment when playing as apposed to your idea.
I get enough 'don't kill steal' business as is even when the overall gold total is higher for a team when everyone hits them as far as I'm aware, which you cite.
I'd rather see less OMG I'M the GREATEST EVAR I deal the most damage to enemy DGs! and more....team play.