Indeed, .na domain registrations and renewals are very high.
Most of the people see it in a different perspective, but shrewd domainers see this with a different perspective.
I'm not going to spend my energy to explain why domain hacks are a smarter choice than traditional domain names. In this thread it has been discussed for so many times. End-users are spending multi-million dollars to acquire pure Geo domain names like China.com.
There is no doubt that .com is KING, but the internet trend is now changing rapidly.
Chi.na would become a vibrant alternative and it would add a creative branding strategy to any business which focuses China.
Considering the renewal and registration facts, as you said, the registration and renewal fees for .na domain names come around $4,500 per year if the registrant is a foreign entity, and there are NO RESTRICTIONS in holding a .na domain name.
FB.com was bought by Facebook almost 3 years back for a whopping $8,500,000, and they are not going to worry about the resale value of this domain name, because they are NOT going to resell it ever. It's one of their valuable business assets.
Spending multi-million dollars upfront for a .com domain is a worst decision, when we have the choice to hold on to a beautiful and creative Geo domain hack for just $450,000 for a whopping 100 years, even at an extreme renewal pricing model of $4,500/yr.
In this $4,500/yr registration/renewal model, the buyer has the choice to pay yearly, which of course reduces his huge upfront costs in getting and establishing a unique brand identity.