Sustainability is really the achilles heel of inflationary economics. The Earth is finite. Exponential growth cannot continue forever. When a steady state is reached, that's apocalyptic to inflationary money systems since debts all the sudden can never be repaid.

Edit: I don't think space colonization matters here. Sure, it may be possible. But Mars is sufficiently far from Earth and the energy cost of transport sufficiently high that a populated Mars would effectively be an entirely separate economy for everything but intangibles. I don't think a Mars colony would permit Earthly exponential growth to continue, even though Martian exponential growth could occur for quite some time. (Until Mars' own unique limits were reached.)