Part of the issue was that replacing the phones was not needed,
getting new batteries was enough (that was initially a 85$ replacement I think, that got down to 29$ after the issue was made public if my memory is correct. Don't quote me on this)
Case in point, when Apple explained the issue and their hack, people got way more interested in checking the state of their battery (wether they were affected or not), and there was a rush to replace used up batteries.
I tried to replace mine 2 months after the announcement, there was still a 3 weeks backlog of replacements.
Also to note: those phones were still turning off in some circumstances. It wasn't like Apple's hack completely removed the need to replace the faulty battery.