The gold generation from the jungler items isn't really all that great. I remember seeing somewhere that if you managed to use up all your stacks consistently it only amount to ~40g per minute. The global gold gains and assist streak bonus gold are the bulk of the new gold out of the jungle.
It adds up. It's not as terrible as it looks on paper because the Spirit Stone final form items are, in and of themselves, pretty expensive (2000 gold vs. 1650 for Wriggle's) but you're still going to end up with 500 or more less gold per game if you were someone who used Conservation even marginally efficiently.
It doesn't kill Feral Flare builds outright, but it has to be accounted for when it's all said and done. You need to be playing a champion for whom the stacking passive of Flare grants equal or greater value compared to the missed gold due to an honestly
terrible gold generation passive (it was kind of shit even when it was 40%, and now it's worse). Most "melee carries" pass that litmus test, but almost all AD casters automatically fail it (not that most of them can live without Spirit Stone sustain, anyway).
Trust me, it does most definitely add up. 30% gold from jungle monsters sounds fine and dandy until you realize you "donate" a lot of your big monsters (ie, the ones worth actual money) and late-game when your carries leave lane you lose the ability to do more than pop in for a quick nip in the jungle almost entirely. That's when Conservation just starts to ridiculously outpace Wriggle's for gold generation, during those times when you
aren't hard-farming the jungle; ie, the entire back half of any given game.
Edit: Although it's also worth noting that Wriggle's ward placement passive is probably undervalued slightly due to having such a ridiculous range now. It's hard to put a price on being able to ward from farther away, but it definitely helps add some more "phantom" gold generation onto the item.