I am fairly certain the 3ds models including xl's and special editions will be phased out so the only option available is the "new" 3ds.
That could happen, but at the same time, if that works only as a replacement, sales won't go up and the software that will only come out for the NEW 3DS, won't have a big userbase to sell on.
They need people to rebuy the handheld so they can avoid bringing a new handheld sooner than they wanted.
Most likely not because of Japan, but because of the US and Europe - where they don't have the same SW support they have in Japan. If i'm not mistaken, NPD sales show a meaningful fall from last year to this year. And this time around 3DS doesn't a fresh new Pokemon or exclusive holiday title (in regards to Smash) to help the console sell during the holidays.