Growth is always unsustainable, The Dude. It will never be sustainable. Sustainable means "able to be sustained". If you need to add, "for some period of time", then add it. Sustainable growth is an oxymoron.

Resources will never be infinite, as your previous post said. Sure, some substitution will occur and maybe even resources thought of as unusable may become usable but, if we haven't already reached there, at some point those substitutions will start to be not quite as useful, not quite as abundant and not quite as easily extracted or processed as the resource it is trying to replace. At some point, we will use, or have used, the best resource for the job, after which, substitutes just won't quite make it.

Also, not all resources considered vital for our societies, or considered vital to maintain growth, will be able to be substituted, or substituted at anywhere near the same utility and scale as the declining resource. So we only need to some vital resource to become scarce for growth to cease for ever (apart from occasional minor spurts, if humans survive). That resource might be topsoil, water, habitable land or some rare metal. It might even be people (since the economy needs consumers, to grow).

The planet is finite. Get used to it. Humans aren't really as smart as you seem to believe.