It will probably sell HALF of the Steam Deck lifetime, if even that.
The Deck was a smash hit, relatively to its niche of reference, and that's because it covered a particular gap in the market (affordable handheld PC) which is something that these Steam Machine don't seem to replicate.
Morons keep comparing it to "Nintendo numbers" as if that was the average standard to live by, but they should try to ask to any new hardware producer on the scene if they'd be happy to launch a new device and ending up in the 4-6 million ballpark and then report the answer back.