I don't think it is a reasonable restriction at all. Besides the most devoted fans, I don't believe many families would purchase multiple licenses to a lot of games just to be able to play them simultaneously. These kinds of terms don't really increase revenue but they unnecessarily damage the experience at the same time. This is exactly the sort of stuff that makes pirating more convenient. As I've mentioned in another comment, the Mac and iOS App Stores don't really have these limitations.

What do you suggest then, Valve should give you unlimited copies of the game when you purchase one?

Valve are not giving away a single copy of anything, they are licensing intellectual property with specific terms. I'm advocating licensing terms that allow licensees to simultaneously install and use licensed items on all computers owned by them. The terms of the Mac App Store already allow this.

This isn't something that's on Valve, but games should bring back LAN play.

I'm kind of surprised that game companies now expect you to pay for a game twice to play through a two-player co-op campaign. Remember when local multiplayer was a thing? When you'd buy a game, and several people would sit around a TV getting "simultaneous value out of it" for the cost of that one purchase, because that was the game's value proposition to begin with?