So why use Arch then? You're not going to use the main feature of it which is rolling release and constant updates.
You're not going to do anything to it. Valve will be the ones monitoring the Arch repos and testing it before releasing it to you. Maybe they want to have the option to have the latest updates, if they deem them stable, rather than having to wait.
Kubuntu is hardly bloated, and if you're really worried about size, Ubuntu has minimal and server images you can base a Plasma-powered desktop off of.