That's right, the price will probably fall quite a lot after the airdrop is finished... that will be good time to buy because then it will probably go back up.
And if the project still lives by the end of the airdrop, there won't really be much of a reason to dump since the price will keep going higher. But if a dump is imminent, it just means that you can buy extra Onions for cheap, so whatever.
You have right. After all the airdrops, it will exist a lot of Onions in circulation. Some people will sell all the coins and others will sell just a %.
Anyway, a lot of people will hold their onions if the dev team can create good features before the end of the airdrops.