I think the author may not be entirely serious, and may even have been attempting to achieve some level of humor or personal catharsis with their writing.
It doesn’t seem like the author is upset about this, just interested in seeing what the eventual outcome is. It is interesting to see the tension in their emails between wanting to get the user back as a subscriber and not wanting to continue to host their files for free.
Companies that offer things for free, I fully expect for them to keep them free forever. If the pay solution is worth it I'll pay.
When they start changing the contract, I find an alternative and use the service as much as I can. My Dropbox is fully backed up but has been full for years and it doesn't matter to me any more.
These emails are automated, so they can send them to many users, and only need a few to come back to make it profitable to do so. Also, the team that is responsible for these emails and for retention (or winning back defectors) may be its own little island and not care about contradicting the forever-promises made elsewhere in the business.
What promise? You know agreements change all the time right? Google photos, gmail, etc all used to be unlimited until recently. They’re running a business on money not promises from some forum reply.