When making such a big accusation or complaint, it would be great if you showed us examples/proof where merit sources have flooded their "sockpuppet" accounts with merits, and they are now milking the hell out of signature campaigns.
As far as I recall, at the beginning of the merit system, there were a couple of merit sources that abused the positions and got removed.
What I am seeing right now is just an assumption with no single proof.
Becoming a hero or Legendary member is not so hard. Even when you don't try so hard. Just be an active and helpful member in the community, you will get there.
What needs to be done and I think the only thing that can be done would be for one of these accused "merit sources that monopolize the campaigns" to get caught red-handed and that they are really sending merits to their alt-accounts to create an account farm so they can maximize what they earn in the forum. But either they're really innocent in all of these accusations which is the likely thing or that they're real slick about this that there's no hope of them getting caught anytime soon. As you've said, proof is what needs to be laid down by OP but given how difficult that would be to do, we can all just speculate, allege and accuse and those things don't mean shit so speculate, allege and accuse to your heart's content.