I think this would be a great time to bring up the old rock/paper/scissors paradigm of bot lane supports.
Back when I started in Season 2, there was a clear understanding with the majority of the player base that sustain beats poke, poke beats engage, and engage beats sustain. That paradigm is not really being upheld with these new items, and I think it's time we start to talk about it.
I think the three gold items ought to be tooled around those goals. And, to be fair, spellthief's edge IS. And that's why it works. The mechanism of spellthief's is tied to successfully playing out a time-tested support strategy: poke.
By contrast, relic shield is this strange frankenstein of goals. It has health regen, which you might think is a sustain item, but what about melee supports that engage? What are they supposed to do? At the end of the day, a little health doesn't help you when you go all in on a ranged ad champ. You're gonna have to flash out if you don't kill them, and relic doesn't help you kill them.
As a result, coin is now this strange frankenstein of poke/sustain. But only mana sustain, because it would be too much like relic shield if it had health regen (I suppose that's the logic, anyway). So now, you really need to be a champion with a heal to use coin, or it doesn't give you enough sustain to do anything in lane (other than die, of course).
Why don't we make relic shield an engage item? And make coin the sustain item? And keep spellthief's RIGHT where it is? Would that not work better?