You are thinking is perfect. This is the way bounty programs shovel be working today also. With the increasing number of participation increases the number of dumpers when the coin get listed! This reduces the token economy badly and thus makes it worse.
Also it doesn't matter if there are 5K participants or 1 ml participants as long as they are scammers then whats the use of spending so much money?
Instead of that they shovel be spending their money on the real development of project and increase the value of coins!
And what if...those airdrops can help people in need? I believe that ICOs are giving away too much tokens for useless airdrops,instead of supporting meaningful causes...so I am doing a little experiment - I created Noble Airdrops to convince ICOs to support the ones in real need. Projects willing to donate tokens for charity will receive exposure on the platform. Check it out:
http://nobleairdrops.org