Just guessing, but might they have had something in their EULA / TOS up until October 4:th 2012 that somehow forces them to keep the Public folder available? Maybe some language about how features are removed /deprecated, or about how changes in the EULA / TOS are allowed to be done, that makes it infeasible to do without breaking the contract?
Maybe just coincidental, but while I was searching thru Dropbox mail looking for details for another reply here, I noticed this message from Aug 26th 2016:
"We’re reaching out to let you know that if you haven’t updated your Dropbox password since mid-2012, you’ll be prompted to update it the next time you sign in. This is purely a preventative measure, and we’re sorry for the inconvenience."
2012 was apparently when Dropbox had 68 million email addresses and hashed passwords compromised. It just took 4 years for anyone to find out about it.
Cynical-me wonders if this is because whoever got this passwords also got the backend source, and they've discovered security problems that're easier to fix by killing features than but actually fixing the code the attackers have?
October 4, 2012 was the cutoff where they stopped generating Public folders for new Dropbox accounts-- at the time, existing accounts could still use the Public folders that had already been created.
I'm guessing they mention those accounts in the announcement to make it clear that the changes don't apply to those accounts, since pre-10/4/12 accounts never had a Public folder at all.
I don't understand why October 4, 2012 is even mentioned here. That's when they disabled public folders for new users, but now they are getting rid of all of them. Dropbox is getting rid of public folders entirely.