Yes, someone would have. Eventually.
But valve did it early.
It's easy to fill a niche once it's formed. Not so easy to do before, or as it's forming. Or predict if one will form.
I'm not saying valve or Gabe had some kind of foresight or wisdom, I still think it was a gamble. It just happens to be a gamble that worked.
I actually hadn't heard of this movie yet. I try not to keep up with movies and games so I'm not too disappointed by over-hyped content.
I got a little excited at just the title, and as soon as I saw "Ares (Jared Leto)" all desire to watch it went out the window, my insides dropped a little, and the Tron Legacy is now derezzed. Pun entirely intended.