Of course you can't build a Dell to match that. The point is this is less of a computer for the longevity that the Mid 2012 Mac Pro originally priced at $2,499 provided. (Guess what, the Apple Thunderbolt Display then was also priced at only $999.) Why in the hell would I pay twice that, for a computer I can't use for twice the lifespan? There are _still_ people selling their Mid 2012 Mac Pros for a large fraction of the original price.
But, no you're right, it's totally acceptable that Apple's only VR-capable machine is $5,000 to produce VR content that no one on the Mac platform can use.
Genius.
Provided you don't go for the lowest spec one, it's probably more than enough to cover most needs of most power users for the next 5 years or so. Even if you go for the low-spec one, it'll be fairly decent 5 years from now just like my aging MacBook Pro is.
You can't build a Dell to match that, even if you rip Windows out and install Linux or a BSD.