You really don't. I have a 3ft by 3ft playspace (at most), and I played through Alyx (as well as tons of other VR games) just fine. Sure, having more space would've made it a little bit better, but the overall experience is just as great.
Most companies when they introduce a platform do try to push it with some of their own high profile content. Valve not so much until recently.
It's only been the past several years that Valve has completely stopped making games, and started working on other things.
It's easy to estimate roughly from the steam data that is publicly available.
Much like music, movies, pretty much all media.
Not that you are wrong, but i feel like it would be more interesting to see those numbers in the context of the entire install base. Like, number of sales compared to the number of those devices in people’s hands.
Big name, but random googling says Half Life Alyx sold 50k vs.... Animal Crossing at 11 million+ by March 31.
I'm not sure Half Life Alyx has any relevance as far as volume of play goes ;)