Tu quoque fallacy.

No, I didn't say that these weren't.

If you want to know, rather than project on me, my beliefs, you could ask me. As it happens, my view is that humans, as with other life forms, exist to perform the function of exploiting low-entropy energy stocks and flows. The consequences of that ... tend not to be something we consider in advance. We've benefited hugely from fossil fuels, but have put ourselves well beyond the point of sustainability. Even with, say, a practically unlimited energy supply we'd bump into the problems of heat dissipation within a few centuries to millennia at present growth rates. We've simply got to stop growing.

As to what's sustainable? Probably on the order of 500m - 2 billion souls if you want any sort of industrialized lifestyle. Hell, even if not, not much more than that.

The next century or two will be very interesting times. Starting likely within a decade or two, possibly less.

First I've every heard of any such proposal. How exactly would you go about stabilizing every last decay product to the level of background radiation?

Your statement is equivalent to noting that there's nothing in chemistry which prevents us from building coal-fired power plants which capture all CO2 and toxic pollutants (mercury, sulfur, particulates, NOX,, radioactives, etc.) as well. However it's economically and thermodynamically infeasible. Not that emissions haven't been drastically reduced from early designs, but it turns out that that's still not good enough.