That's fair enough - it's my assertion versus your assertion 🙂
However, let's just examine a possible scenario. We can agree that Apple is in business to make money: indeed their shareholders require it.
So: let's suppose you wish to store 1TB of data online for a year and then retrieve it. In the normal scenario you would pay your $9.99 a month for the year - $119.88. This isn't a bad price - the same as Google Drive and considerably cheaper than Amazon S3, for example.
Now let's suppose you pay for one month, upload your data, then cancel the subscription. A year later you pay for one month again and, under your assumption, your data is there and you are able to download it. So the exercise has cost you $19.98 instead of $119.88. The logical question arising out of that is, why would Apple permit that?
But as you say, all this is deductive reasoning. Perhaps one day we'll hear from someone who has tried it. I would hope they had a backup elsewhere before starting, though.
I'm happy to keep you happy by rewording my previous post:
'It's reasonable to assume that data occupying storage above the 5GB will be deleted - it's not possible to predict exactly what data would be removed. It's not been stated whether there is a 'grace period' - most likely there is a short one but I wouldn't rely on there being one if I were you.'