You are the one that is acting like it's a forgone conclusion, I even admitted earlier that a decline is possible but it's certainly not set in stone like u are making it out to be.

As long as Nintendo can maintain a 50-80 million baseline for future handhelds than they certainly have a future.

The only thing that would stop the next Nintendo handheld being a certain buy for me is loss of backwards compatibility.

As a result I think Nintendo are tied to the basic ds design now.

It's novel design also has the bonus of making ds emulators on phones hardly worth wasting time with. If it was designed more like a vita most games could play fine on a phone with bluetooth joystick.

It's entirely possible yes. PS4 might get 120m, but that would still indicate a downward trend. If PS5 did 110, and PS6 did 115, and PS7 did 105. That wopuld still indicate a downward trend.  More fundamentally though is percentage of sales of an entire pool of potential customers. PS1 was not alone in its gen, there were others. So far there is every indication that the Mobile Gaming pie is shifting ever more towards non-dedicated hardware to such an extent that even with an increasing population there are less and less number of people bothering with dedicated mobile gaming hwardware.

You may or may not argue that GBA was alone, but you cannot argue that DS was alone.  The DS gen was the pinnacle, combined sales of DS + PSP > 200m, the current gen 3DS + PSV is going to be lucky of they hit 100m. Where did all the sales go? They went elsewhere or dumped mobile gaming altogether.

Tell me where on earth you think Nintendo is going to go UP from here? 3DS by many measures is the best Nintendo handheld ever if you are talking strictly about the games.

I think more and more games are going to come out on the platform with the most hardware and thats mobile phones/tablets. It's a tragic shame but there it is. 

There needs to be more than a single data point to determine a trend, handheld gaming rising for multiple generations in a row followed by a single generation of decline does not signify a decline, if next-gen does worse than this one then yes, we can start talking about a trend but as of now this "trend" does not exist.

GBA-81 million, add in lesser competitors such as N-Gage, etc. about 85-90 million

DS-154 million, PSP-81 million, add in lesser competitors, about 240 million

3DS-lifetime estimate 65-70 million, Vita-lifetime estimate 15-20 million, about 85-90 million

So yes we have seen a huge decline gen-over-gen, can such a decline continue? Certainly possible, maybe phones/tablets are simply going to kill off dedicated handhelds or maybe 80ish million will be a standard baseline for future generations like the 6th & 8th gen, or maybe Nintendo comes up with the next big Pokemon, Wii Sports, Minecraft style craze that causes millions of people to rush out and by the next handheld. My point is that the future is uncertain, none of us know which direction the handheld market will take, it could go up, down or simply stay still, we need to wait and see how it plays out instead of jumping on the "dedicated handhelds are dead!!!!" bandwagon.