I wouldn't call something a threat if this thing is an evolution of its precedent. Virtual Reality casinos are not a threat to online casinos because think about this, you'll need internet to access VR casinos. Since you need internet to access them, they'll be a part of online casinos. Some casinos might try to create a dedicated VR casino but I think that long-term, casino game suppliers will create VR games, including VR slots, VR live blackjack, Roulette and so on, and these providers will offer their games to different casinos, they'll implement them and then offer VR experience to their users.
I don't know how possible it will be for a casino to have only Virtual Reality option. Just as you highlighted, this option might be added to the services provided by already existing casinos. The younger generation might be interested in this new invention but I don't also see it as a threat to online casinos. Bettors will always choose normal online games because it's not everyone that would be interested in VR experience.
It's pretty much simple. There are crypto casinos and there are fiat casinos. What stops fiat casinos from accepting cryptocurrencies? Imagine LeoVegas or bet365 starting accepting Bitcoin transactions, lots of casinos and sportsbooks would lose customers.
Exclusively VR casino is very much possible. This will be a casino focused on VR experience that will try to implement its VR games on as much devices as possible like Smart TVs, Smartphones, Desktops and so on. They'll have customers who want VR experience and nothing more, just like Bitcoin customers that play on crypto casinos because they want to deposit and withdraw with Bitcoins. But I think that long-term, such a plan won't be the most profitable plan because it's clear that creating a product and renting it to other casinos works way better.