I'm very excited about the Steam Machine for the reasons you mention - I want to buy a system, not a loose collection of parts that kind-of-sort-of implement some standard to the point that they probably work together.
There's nothing wrong with ATX or having interchangeable components. An established standard means that small companies can start manufacturing components more easily and provide more competition. If you turn PCs into prepackaged proprietary monoliths, expect even fewer players on the market than we have now, in addition to a complete lack of repairability and upgradability. When you can't pick and choose the parts, you let the manufacturer dictate what you're allowed to buy in what bundles, what spare parts they may sell to you (if any) and what prices you will pay for any of these things. Even if you're not building custom PCs yourself, the availability of all these individual components is putting an intrinsic check on what all-in-one manufacturers can reasonably charge you.
There are systems like the NUC but if I want a super-high-end 5090 and top-end CPU, all of the options to cool them feel like... well, something kluged together from whatever parts I can find, not something that's designed as a total system. Maybe we'll get some interesting designs out of this.
I'm really fearful that PCs are going down the road of locked bootloaders, running the user-facing OSs inside bare-metal hypervisors that "protect" the hardware from the owner, etc.
I'll accept that I'm likely under the influence of a bit of paranoia, too.
I'm strongly of the opinion several unaffiliated factions (oligarchs, cultural authoritarians, "intellectual property" maximalists, software-as-a-service providers, and intelligence agencies, to name a few) see unregulated general purpose computers in the hands of the public as dangerous.
I don't think there's an overt conspiracy to remove computing from the hands of the public. The process is happening because of an unrelated confluence of goals.
I don't see anybody even remotely comparable in lobbying power standing up for owner's rights, either.