Pirates that want to gain then maintain territory, as long as it's not near Marine/WG installations aren't a threat to the WG. Imu and the Gorosei don't want One Piece to be found. They don't want the Ancient History to be understood.
Rayleigh said they were "too early". I think that meant it wasn't the right time, but, also, that Roger's crew didn't know enough.
Random warmongers finding One Piece isn't even that dangerous to the WG.
Dreamers like Luffy, who want to change the world, rather than rule it, are threats to WG and IMU. The Yonkou serve as a deterrent, a disgustingly powerful force to keep people from Luffy, who want to change the world, from reaching One Piece.
That's why the Gorosei are making their move now. They can't sit back anymore. Even Shanks is going after the One Piece. Luffy, Blackbeard, Shanks, and Buggy, the Yonkou of the era, have no problem moving around. They don't want territory, they want strength. This new era is AWFUL for the WG, because it's full of powerful pirates WHO ACTUALLY ARE MOVING TOWARDS THE ONE PIECE.
Kaido and Big Mom were chasing Roger's shadow. Whitebeard was never in Roger's shadow, and didn't care for the One Piece himself. The Worst Generation defeating the Yonkou and opening up the race to Laugh Tale is the worst case scenario for the WG.