Well, I guess as part of a larger preservation movement we'll need to reverse engineer the system and eventually go on to emulate as well providing the community cares enough ...

Huge differences. 

The 3DS is also bad compared to the GameBoy and the GameBoy Advance. 

And the explanation for the lower sales during the PS3 era is simple: competition. The PS2 generation sold in total around 200M consoles, and the PS2 crushed the very weak competition with 160M. The PS3 generation sold 260M in total, so a lot more, but it was around 100/80/80M, no-one crushed the others. PS3 gen > PS2 gen. 

The 3DS, on the other hand, has no direct competition compared to the DS. The DS generation (DS-PSP) sold 230M, the 3DS generation sold 90M. Even without competition, and even if it took 100% of the market, it would still be a huge fall. 3DS gen <<<<<<<< DS gen. 

It's not "The DS was a monster, you can't compare", it's that the entire handheld market attained its peak at that time and is falling hard since then, mostly because of mobile gaming (which is exploding further every year).

And now, only the Switch kind of remains on this market, with home console playability to add interest. We'll see if they sell more than 90M consoles to stop the fall.

The home console market attained its peak last gen, it will be down this gen, and maybe even more next gen. Mobile gaming doesn't only affect handheld sales, especially in Japan. 

Yes, in the past I thought it could reach 25M if Sony didn't almost totally neglect it (the worst omission was not securing Monster Hunter and other 3rd party games that were big system sellers for PSP, probably with all the biggest PSP 3rd parties it could have sold ~1/2 of 3DS sales, just like PSP did compared with its competitor, DS), and with two well-timed price cuts. A wasted opportunity, but probably, before PS4 success, Sony could dedicate enough resources only to try and get back the leadership on home consoles, using more than it actually did on portables would have had a far worse risk-benefit ratio that Sony simply couldn't afford back then.
Even with this scarce support, PSV has become the second best selling non-Nintendo portable console, but it also consumed the PS brand strength in that market, now only including a hybrid in PS5 range will maybe allow Sony to get a slice of portable market, much smaller tham Ninty's one, but large enough to justify the investment. Doing it this gen is impossible, only 7nm or smaller process and next gens GPUs will offer the power Sony needs, not to mention that rushing it could cause design flaws and show weakness.
So well, what can I say, gratz PSV and Sony about this niche milestone and having found a profitable balance in this niche.

Since Intel retired from that market, it's very unlikely, as even their Core M processors (the Y series) are too power-hungry for a handheld.

Grats for the milestone. Sadly it will probably never reach the 17M milestone (or take an eternity for it).

At the rate it's selling currently, it would take about 6 years to reach 17m, so yeah, probably not.

In Sony case, yeah. In Nin case, as long as Nin portable still have full Pokemon/Animal Crossing/Mario Kart/Super Smash Bros and all Nintendo proper first party games while mobile phone only have things like Fire Emblem Heroes, Super Mario Run and Pokemon Go, their system will be fine. 

My previous comment literally proved the opposite. The market crashed with a 60% fall. Nintendo resisted better, sure, but still crashed with a 50% fall. That's not "fine".

If a market is shrinking, the worst thing the leader of this market can do is saying "We're the leader, we're strong, we'll be fine" like you're saying. That's how companies, even strong ones or giants, go bankrupt. 

If the handheld market keep shrinking, Nintendo will limit the damage for a while, sure, but they will just be the leader of a niche market with only the most hardcore fans, a shadow of the past. 

Why do you think they started to release mobile games? For fun? No. The market is shifting from handheld to mobile, so they have to follow it. And they will keep doing more and more mobile games in the future. 

It's also why Sony is working on game streaming, Microsoft on services, and why both Nintendo and Sony went from 2 consoles to only one. Handheld are the first to go, but home consoles are also shrinking, so all the actors are preparing for a shift in their model, at least in part. 

Good, let's compare Fire Emblem Heroes to Echoes Shadow of Valentia, Super Mario Run to Super Mario 3D Land, Pokemon Go to Ultra Sun/Moon and yeah, totally same quality.

People comparing sales of older consoles to the more recent ones is like comparing apples and oranges in a way. The PS2 and DS was from such a different era for their time when there was none of the stuff that exists now or were new for the time; there was no smartphones or tablets at the time for the DS to compete with, so of course the 3DS was going to suffer somewhat in an era of smart devices. The PS2 sold well, partially due to it being a DVD player when at the time such a thing was pretty expensive and costs almost as much as a PS2 itself; Sony screwed up the PS3 badly with the comparatively high cost and being so hard to develop for which resulted in the lack of decent games for a long while, despite the fact that it was much less expensive than a dedicated and obscenely expensive Blu-Ray player.