Why on the Earth should I worry about it selling 5 or 15 millions units? I'm a gamer, not a Sony investor, and since even at launch there will be games that will amaze me, and obviously more planned for years, I already know that I will have what I want for it.
What's the sense of basing the decision of buying or not on how mant units it will sell in the coming years? It's not that that's will change the games that are being developed. And surely I'm not thinking that PSVR 2 will have to last two generations. As the console, all I need for it is to last for the entire gen and some, with some meaningful games. Since the architecture, it will probably even back compatible with PS6, but as for PS6, at that time I will buy PSVR 3, if it will exist, to enjoy another next gen step of it.
PSVR 2 selling good or bad can only influence a potential PSVR3, but at least for this generation, we are pretty much covered. There's always a years long schedule, internally.