Asiasoft's definition of "permanent".

The permanent version is different item than the time-limited version. They have to do a couple things to make it happen:

1) patch the quest to give a different reward.
2) replace the ones that have already been handed out

The issues with each of those -

1) They need this coded and a mini-patch issued. This kind of change request may not be able to be scheduled and/or may take longer than the event period to fix. It -seems- very simple, but it's still a hassle.

2) They may not have a way of determining which temporary chairs came from other sources and events and thus if they replaced ALL of the time limited chars, they would be upgrading chairs that were never meant to be upgraded. Again this sounds silly from the outside, but it is still probably a concern.

2) It's very easy to tell which chairs came from the event. The ones whose expiration date is exactly 30 days from the day the character completed "Finding First Loves - Day 8" (or w/e the name of the quest for getting the 8th day reward is). It's probably safe to assume that any PB chair which expires between 20-28 April is from this event, but as I said, if you want to be certain, check the quest log.

1) There's no need to fix the quest. They can simply issue notice that "The chairs will be replaced with permanent ones in a maintenance on April 4th. Please complete the event as it is for now." Of course, we'd have to believe them...

The only real technical problem I can think of is that there is no permanent version of this chair in the data right now, so they really don't have any perma PB Chairs "in stock". They can't put something in our inventories that isn't in the data (without a patch).
However, they can get around this by taking a page from Asiasoft's book, and making the chairs "perma" by setting their expiration date a few decades away.