I think Splinterlands does a great job of incentivizing the continued purchase of packs by attaching value to the act of purchasing a pack through airdrop points. The more packs you buy, the more airdrops you are likely to receive, and there are multiple.

Imagine if GU had a set of X promo cards that were released each week, one at a time, as an airdrop to those who bought packs. I'd certainly be more willing to buy at least a few to get a couple airdropped cards ;P