Personally, I understand the appeal of the Steam Machine, it's also cheaper than a standard prebuilt which helps. The fact it can basically be a nice, cute form factor machine is great! There is room for optimization in the future to make it run games and shit better too.

Personally, I would rather buy a prebuilt over putting a computer together myself because...While I can get a distro installed and use the package manager and command line to set stuff up. I am ever afraid of actually plugging and screwing things in. As my goose on the loose energy is LEGENDARY for causing random fuck-ups. Random sneezes, itches, all manner of odd occurrences will distract me to the point of not being able to reasonably complete the build correctly. I can change thermal paste, and do little things. Nothing as big as making my own computer from a bunch of parts boxes.

Edit: I still might try and fix my busted BIOS PC anyway, but, first I will get a mini-PC that will be a guarantee that I can play some games that I have been missing out on.