Isn't it? If I have things right, then, without them you'd largely be relying on a wealthy benefactor. Outside of huge, prestigious leagues in large countries -- like the EPL and NFL -- I'd imagine that's not a very stable long-term strategy. Indeed, I'm pretty sure that point has been alluded to on this thread by Australian posters.
On the rugby playing side of things, isolationism may guarantee Australian wins but will tank the Wallabies.
Well, if there's one culture who could endure it, it's the Japanese. The Sunwolves crowds have been excellent even in some of the matches they've been easily outclassed (the number of which seems to be decreasing). But, more seriously, there's nothing to say that loans and the like couldn't be put in place for any theoretical non-Super Rugby competition.
Do you? I'd argue that they're only there because NZ can field a full conference worth of competitive sides by itself. Outside of that, the NZRU played a big role in securing the Sunwolves spot (IIRC), shielded them from the cut and were the ones who originally brought big tests to Japan (vs Australia, 2009; vs Japan, 2013; others forthcoming, I believe). I dare say they have plenty of goodwill with the JRFU. Plus they have the All Blacks brand.