Coin basically removes the counterplay for passive supports
Before the inane buffs it went through you could rely on causing a passive support to use up their mana and open up good opportunities to engage, but now that the item is literally infinite mana for the low low price of 350g you watch a passive support spam their skills then be at full mana in the matter of picking up a few coins
The mana coin portion is something more suitable for a fully completed item, not a starter item And remember the mana coins are the "2nd passive" of support gold gen items, so compare infinite mana to the tiny bit of damage Spelltheif gets or the small heal that Relic gives, Coin absolutely blows both of them out of the water with that power alone, not forgetting that it nets you more gold on average then Spelltheif or Relic procs
Coin was clearly overbuffed and it really needs to be brought down a number of pegs, just because it has a history of being bad doesn't mean it should have a stanglehold over the other gold gen items
need to give a lot less (something that would mitigate the mana famine some go through) or not at all and only at further upgrades. I like the item but too OP