You can run media from a potato. This will be at least 5x the cost of a cheap mini-PC. And you can't forget power draw. The Steam Machine has a 30W CPU, and I'd guess about 60W RAM which would add up to something like $120+ annually where a mini PC would cost closer to $7. 5 years later and the Steam Machine has eaten it's cost in power. Assuming it costs $500 like most consoles, you are looking at a total 5-year cost of $1100 where a mini-PC would be $100–200.
I think the mini PCs they're talking about are more likely to be N100 systems or similar that are sub 100 dollars new. Significantly less anemic, and their hardware media decode (which is well supported by software) is more than sufficient for realtime 4k playback.
Media box under your TV? Right now I don't have a lot of options that also don't inundate me with ads.
Sure, I can build one, but if Valve can put this out at a price that makes me go "Nah. Not worth building it myself." that's a win.