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.
Did...... Did you down vote your own comment? I'm still not sure how federated votes work, but for me on startrek.website it says 0 up 1 down.