The current Steam Deck is more of a proof of concept, probably if other manufacturers and Steam want to continue it it will just create a new kind of PC hardware sector, much like how you have tower PCs, laptops ... this is kind of like a hybrid of the laptop and tablet computer. Maybe in 3-5 years you'll be able to walk into a Best Buy and there will be several models of this concept sitting around.

3-5 million

Those are some big-ass increments in the poll. It reminds me of old cars that would reach 60% of the speed shown as max on the speedometer. Valve have a history of axing, as well as misjudging their aim for hardware, and the product seems very niche. I think it will be loved to bits by a very small minority when all is said and done. And I still think those poll increments are ridiculous.
If I had to pull an actual number out of my ass, I'll say that we should celebrate if it beats the Wii U - at least in its current form. If it gets many variants and manufacturers behind it, like laptops and phones, this could go up considerably. But I don't see why other companies would jump on this, given the sheer breadth and number of potential gaming devices already available on the market today.

I decided for the poll increments to put my personal expectations aside. After all I am far from being a predictive genius. So I was assuming, the Deck could sell anything earlier handhelds did. After all it is the most powerful handheld ever and has the biggest library ever right from the get go. These are usually good signs, right? So I did go with the highest selling handheld and gave some space for the possibility the Deck even overtakes that, so I arrived at 200M. From thatn I parceled it into neat equally sized portions. I think that is fair.

There are some key factors in that which I would disagree with though. The first, and foremost, being the power assessment having any relevance at any device's chance of success. The original Xbox was by far the most capable and was marketed on having an incredible processor, the N64 was also more capable than the PS1 in certain metrics. Sony handhelds have also trumphed Nintendo's by some margin.
The second, and rather big, objection I would have is the change in the market overall. There are hundreds of millions of handheld gaming devices out and about every day, in the form of phones and tablets. dedicated handhelds have taken a huge beating since the advent of smart devices on the market and the demographic divisions are completely different today than back in the DS' heyday (the last handheld that sold truly massive numbers). Many prefer proper simplicity on the go, and this also ties in with the library issue; how much is the world's biggest library worth if only a tiny fraction of it is suited to handheld type gaming in possibly noisy, cramped or overall stressful environments? There are other devices that do simple handheld gaming better, and much cheaper. For bringing a machine with you and using it a semi-mobile device, there are still laptops, which also exceed the Steamdeck in every other aspect of utility.

I'm fine with the poll itself, it doesn't bother me, but I don't understand the reasoning behind it. I might also be way off in my predictions, heck, I have been on many occasions. But I would create polls based on market reality and the state of actual things, history is out the window, as the previous two console generations would have shown us already. I'm excited to see what happens though, I learn the most from being wrong. 

Depends on what we call the Steam Deck and whether it will have refreshed and better editions coming out. Like PC the steamdeck is in essence a single generation, so it could be over 100 million eventually (15 years) or just 5 million if they don't make other editions aside from this one.

Because it's not worth the work.