The detail is that you have two people: one is paying for the product and one is not. The one who is paying shares a file with the one who doesn't pay. Dropbox provides a worse experience for the person who doesn't pay.
This is in contrast to services like Zoom or Google Drive where part of the sales pitch is that you get a seamless experience when communicating with people - even if the person you're talking to might not be paying, you can guarantee because you are paying their experience won't be impaired, at least for the duration of your interaction.
User retention. Keep people in the platform so they can't take their data. Force users to come back to dropbox again and again (to run up your engagement stats) instead of throwing the file over Slack or whatever.
