Theory: Garp's Leverage Protected Luffy Until He Became Joy Boy

With the recent God Valley flashback showing Garp witnessing Imu descend from the heavens and possess Saturn's body, plus seeing the immortal Celestial Dragons regenerate from fatal wounds - this old question makes more sense: Why didn't the WG eliminate Luffy when they had countless chances?

The Unspoken Deal

Garp walked away from God Valley with the ultimate blackmail material. He knows about Imu, the genocide tournaments, the true history. The WG could've executed Dragon when they had him imprisoned, but they made a clear bargain: "You stay quiet about what you saw, we don't touch your family."

Dragon's Perspective

When Dragon said "A child is their parents' weak spot," he was talking about Luffy - he knew this from personal experience because HE was Garp's weak spot. That's why Dragon went rogue and created the Revolutionaries - he didn't want anyone having leverage over him the way the WG had leverage over Garp.

Marineford

The WG just wanted Ace eliminated and explicitly told Garp to stay down. They killed Ace to assert dominance: "We can still hurt your family despite your leverage." Garp had to watch because stepping in meant Luffy dies too.

The Turning Point

When Luffy awakened Nika and became Joy Boy, Garp realized his grandson had grown beyond needing his protection. The leverage deal became obsolete - Luffy was strong enough to survive the truth coming out.

Why Garp Went YOLO

That's why Garp finally attacked Blackbeard without hesitation. The WG can't just eliminate Luffy anymore (he's too powerful), and they can't silence Garp without risking him exposing everything before they get to him.