I think the core problem is that you lose that available space even if you paid out through the month/year. I have a suspicion this is an intentional dark pattern to keep people from downgrading and not some weird engineering constraint. So it’s a bad and maybe unethical user experience.

Why couldn’t it be: “Thank you for using Dropbox, you have access to your 1t (or whatever) until X date, after that your files will stop syncing unless you do Y. Come back any time!”