Simple enough: How much units do you think will Valve sell over the lifetime of the Deck?

Current numbers are hard to come by. Estimation put the number at about 1.5 million at the end of may:

https://www.msn.com/en-us/news/technology/how-many-steam-decks-were-sold-as-of-may-2022/ar-AAX1j7W

30 to 50 million
Depending on how long the system lasts for
Or if valve decide/are able to, sell the system at retail like the big 3 I can see it doing higher

I expect low sales, if I am honest. I don't see console gamers being that interested and I would guess most PC gamers want a far more powerful console.

If there was an option for less than 5 Mil, I've gone for that. Steam Deck is a niche pc machine, they should have named it like that.

What are current numbers for Steam Deck? Why VGCHARTZ doesn't track It?

Well they've already more or less confirmed that there will be a "next gen" version of it at some point, but assuming we count that as a seperate system from the current one I would definitely go with the lowest option here. I'd be suprised if it reaches even 20 mil, but I also don't think they expect to do the same kind of numbers as the big three, so that wouldn't necessarily make it a failure.

The problem with this is that Steam Deck isn't a traditional handheld... It's a PC. And because of that and it's been confirmed by Valve that Steam Deck will have a successor sooner than later like every other PC hardware. The Steam Deck isn't something that will have the same hardware for 5-7 years but rather a product that will get iterations relatively frequently compared to consoles. So most likely, at the latest, we will have a Steam Deck 2 in 2024 imo because there's new PC hardware revision every 2-3 years. That is not to say it won't be supported for long though because remember... It's a PC and in terms of software/games support, PCs have a long lifespan.

Then you combine that with the fact that Steam Deck is only being sold through Steam and not consumer facing stores like Bestbuy and such along with limited regions (Japan, South Korea, Taiwan, and Hong Kong only started reservations last month). Then you have the manufacturing capacity of Valve being significantly lower than console companies because this is the first time they had a hardware device being this successful.

So the amount of sales should be fairly low. Most likely, 5 million but I wouldn't be surprised if it's lower. But this isn't a failure that some might suggest it would be because the sales have already exceeded expectations from what Valve was expecting. I do think as they continue reiterating with Steam Deck 2 and such, the sales figures will continue to grow gen on gen.

Are we ever going to know?

Anyway, I agree that this'll be niche. Less so than say something uber-niche like Ouya because of the brand name and the novelty of playing PC games on a handheld, but I think they could be happy with say 10M. Voting the lowest option.

Below 5mil. Expecting the next device to release in a few years.
Considering estimates vary so wildly I honestly doubt it's reached 1mil by now.

I would feel pretty comfortable placing it under 10 million, and under 5 million wouldn't surprise me.