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.
It's a neat game. I've put a couple dozen hours into a few villages over the last few months. There's still a lot to improve on, but based on the little I've learned about the developer this seems like more of a passion project than a money maker.
Chug along, little dev, there's dozens of us waiting for updates! dozens!