Sure, people can use the presets, but on a lower end machine, those aren't perfect for getting the most out of your specific hardware.

If that's the case then you could play around with individual settings to find something you're comfortable with. I've been doing that since I was 12...

Sure, he could pay someone to upgrade it for him, but at that point it loses a lot of the benefits that come with being able to upgrade in the first place.

What? No. The main benefit is that you don't need to throw anything away. You can even sell the comonent you're replacing. Would it be more expensive than learning how to do it yourself? Yeah but it's still cheaper than buying a new computer. It's also not as wasteful considering that a broken cpu would mean throwing away the entire motherboard on a Steam Machine, whereas you would simply replace the cpu on a normal computer.

The Fractal Terra is actually larger than all modern consoles. The Fractal Terra is 11.4 liters. The PS5 is 7.2 liters.

Nope. Fractal Terra is 10.4 liters and the base ps5 is also 10.4 liters. I would even argue that the form factor of the terra is better. The series X is smaller, true but it's only 6cm smaller in one of the sides of the top and 4cm shorter. I really don't think that's that different.

The steam cube is a lot smaller, sure. It's also, like I already said multiple times, completely custom and impossible to upgrade. Not worth it imo. We also haven't discussed the hardware. It comes with 8gb of soldered memory for the gpu. You can see here that that was not enough vram (even at 1080p) for some games over a year ago.

Knowing "how to use one" is different from knowing how to build and upgrade one.

Which is precisely why I specifically referred to people knowing how to use it, not build or upgrade it.