The typical answer when people ask why Dropbox doesn't have a cheap low tier is that the more expensive plans are more likely to be underused, and therefore more profitable than a 100GB plan that users constantly max out.

But I'm also curious about whether they've studied the long-term growth impact like you mentioned. I first needed to pay for cloud storage as a broke college student. I'd used Dropbox's free tier in high school and only needed a bit more space, and I certainly didn't have $120/year to spend on it. I ended up switching to Google Drive's $2/month plan and never looked back at Dropbox. If Dropbox had offered a comparable plan, I would have stayed and ended up upgrading to the $10/month plan when I got my first job. Looking at how much data I'm using right now, I would have become exactly the type of underutilizing user they want.