You have point on that, but bounty campaign is really helping to promote the project by retweeting a tweet or simply share facebook post sure will help a lot, and you have no idea how much people joining in it. In other hand, yes airdrop really attract more people like you say but its only spammers who dang share his refferal link everywhere. But not all airdrops need that affiliate or something like it. Some airdrops really just giving away his token to user and build a strong community, ever heard of blue token ? its 100% airdrops.
Bounty campaigns are way different then airdrops! Bounty campaigns require people to promote a project, talk about it, share it to people but airdrops are simply existing for no reason. I just checked out a website airdropalert and it shows the airdrops where people get coins for just being in their telegram group. I wish there was a stat to showcase how many airdroppers actually invest in the project and i am sure it won't be even 10 percent. A project starts with 100 or 200 legit members but with airdrops we see now 20k or even 50k plus telegram community. That is bonkers!