I think it's more likely there will be comfort or motion sickness issues if you don't get a perfect fit. Some journalists had both issues at the demo, and were told there will be significantly more fitment options next year.

It says you can still buy online. It might genuinely be the logistics of custom fitting taking enough time to flood the store if they don't space them out specifically. I've seen Apple stores make you sign up for an appointment for regular shopping, too. It was at some point in Covid with increased occupancy restrictions, but they did have them. I could still walk up to the store and get an appointment, but it would be a while later.

As hyped as I personally am for the Vision Pro, it seems like a lot of reactions have been mixed. Most people don't really understand just how much tech is in this thing to justify the pricing. (I'll personally argue that it's one of the most aggressively priced things they've made). I'm really interested in whether it's actually a home run out of the gate or whether it takes time for people to get it. I do absolutely think the eventual cheaper version will make silly money once Apple does their thing to really sell it hard, but I'm less sure about the first pro version.