The issue is not hard to see.
We only know about one exclusive game. But, do you seriously think Nintendo doesn't have others in the work in order to push the new model? That's assuming both would be sold along side each other. The 3DS was not doing as well as the DS, but it was doing fine for its own. As a result there are plenty of people who recently purchased it. Now we learn that there is a redesigned, or even perhaps successor, to the 3DS just four years later.
As well as the current 3DS is doing, many feel like it wasn't time yet to force them to upgrade to the new model in order to continue playing the expected-Nintendo first party offerings. I suppose though that Nintendo can be defended in sticking to shorter cycles for handhelds in the past (in regard to GBC->GBA and GBA->DS)
Admittedly, this does look like a GB->GBC type of progression. Technically two gens in one. However, this still affects recent buyers of the current 3DS model.
e=mc^2
Gaming on: PS4 Pro, Switch, SNES Mini, Wii U, PC (i5-7400, GTX 1060)