We had a specific audience in mind when we chose 2 days. We were targeting users who just needed to debug their apps, or a low traffic production app. For this, 2 days is plenty. The space is there mainly so they won't have to think about that 5GB/month may equal to 160mb/day or if a bug floods their logs and pushes them over their quota or dealing with overages.
If you care about retention, chances are you're also building a serious product and you want to rely on LogDNA to be around. For that, we hope you'll be on a paid plan.
We want to build the best product we can and by charging appropriately, we can afford better people.