Making this thread based on the somewhat shocking 3DS holiday numbers Nintendo announced today.
The console is at 50.4m shipped LTD having shipped 7.67m in 2014, its 4th full year on the market. That's also down 5.22m YOY! Over 40%. (At the same year in its life DS was UP yoy for reference).
Regardless of New 3DS, it won't be hitting pre 2014 heights again, at best it can hope to achieve parity with last years sales (but with new 3DS already out in Japan) and no idea how it'll fare in Europe and NA in the long run it's hard to say if even that will be the case.
In the same point in its life DS started dropping this year onwards, 09 (15) saw a small ish drop, but then 10 (16) saw a 33%~ drop and then 11 (17) sales dropped like a rock.
A similar scenario for 3DS looks like this.
2014: 50.4m
2015: 58m (7.5m, down a little yoy)
2016: 63m (5m, down 33% yoy) new handheld likely late this year
2017: 65m (2m, down 60% yoy) new handheld definitely out by now
2018 onwards: 66m (1m~)
Forecasts of 81m (PSP's LTD) do not make logical sense at this point. It would require it to be flat for 2015, 2016 AND 2017, and then NOT drop off harshly with the new handheld debuting in that time frame.