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…
Do they give you a pro-rated refund for canceling? Otherwise (unless I'm misunderstanding your comment) that seems like they aren't giving you what you paid for.