The key mistake of GP and yours here is this part of GP quote you elided:

> regardless of how much energy it requires (up to a point, and assuming clean energy sources)

The world isn't yet powered through clean energy sources in any significant way. Electricity production runs mostly on fossil fuels, and then electricity is currently only a part of the equation - heating and transportation is still mostly powered by burning hydrocarbons pulled from the ground.

It is not just valid, but very important, to consider sustainability in context of system at the whole. Greedy algorithms don't work here. A glass bottle may be "infinitely recyclable", but if doing the actual recycling involves using energy from unsustainable sources, it's not a sustainable behaviour in itself. Sustainability is an end-to-end thing; if any link isn't, the whole chain isn't.

Now that we've established that glass recycling isn't currently a sustainable practice, we can still consider whether it's better or worse than alternative unsustainable practices. Regular "single use, thin plastic bottles, destination: landfill" approach seems idiotic. But alternatives involve "thick, refillable plastic bottles" (something I recall Germany used to do), or "single use, thin plastic bottles, destination: power plant" which isn't as dumb as it sounds - if it could be made the primary fuel source for said power plants, it's a step up. I reserve judgment on the alternatives, as I have no data. And it's kind of a problem today: regular people don't have an easy way to review end-to-end performance of such strategies. So they vote for convenience, or for whatever looks ecologic.

Burning hydrocarbons has still place in a sustainable future - they're a great high-density energy store, after all. But to make the chain end-to-end sustainable, we'd have to be using clean and sustainable sources to take back all the emissions and to manufacture more hydrocarbons.

EDIT: And to be clear, I'm always biased in favour of reusing and (then) recycling. But the energy calculation has to make sense, because sustainability is in big part an energy management problem (the other big part is habitability management problem - not doing net damage to the ecosystem; but, assuming the will to do it, it also quickly becomes a subset of energy management).

Right, I'm talking about technically feasible solutions. And it is entirely feasible to power the world with clean, renewable energy. We have the solar capacity to meet hundreds of times our current energy capacity (using less land space than our existing roads). And over 530,000 potential pumped-hydro sites worldwide to store that energy (of which we'd only need a tiny fraction). For example, there are 22,000 sites in Australia, of which we'd only need 20. The research shows that cost will be a non-issue.

http://energy.anu.edu.au/news-events/anu-finds-530000-potent...

Everything else comes down to barriers in the economic and political domains.

But we don't currently live in a world that's powered by renewable energy, we live in a world where the vast majority of energy is generated from fossil sources. Making that transition will require decades, billions in investment and a lot of challenging trade-offs.

Here in the real world of 2019, energy is the dominant factor. If we need to dig up landfills to recover materials in 2100, so be it - right now, we have a ticking clock to mitigate the worst effects of the impending climate crisis. We simply cannot afford to waste time on wishful thinking and idealism, we need to cut our carbon emissions hard and fast.

"If we need to dig up landfills to recover materials in 2100, so be it"

You were talking about burning plastic bottles a minute ago???

Agreed we need to minimise energy usage and move away from fossil fuels as quickly as possible, plastic usage risks creating a market for fossil fuels, where petrol/gas become cheap byproducts, so it isn't a straight forward trade off.

Yup.

The way I see it, we need to optimize our energy use and ecosystem damage output, until we can switch over completely to surplus clean energy. After that, a lot of ecology talk stops to matter. For instance, with surplus energy put to use in collecting, recycling & emission capture, we could use disposable plastics to our hearts' content.

Unfortunately, as you say, widespread deployment of clean & sustainable energy sources comes down to barriers in the economic and political domains. That's why I feel modern ecology needs to optimize for reducing energy use, even if it might mean preferring some locally suboptimal-looking solutions that turn out to be globally more efficient.

"The key mistake"

Not really because I wrote it.

I could have written 'glass recycling has the possibility to be sustainable', but instead I wrote 'glass recycling is sustainable assuming X'

You could complain if I didn't state my assumptions, but I did.

I currently buy 100% green electricity, I could use that to recycle glass today and that would be sustainable. So this isn't some hypothetical, it is possible today.

That isn't to say that I disagree with your points though. It isn't an easy problem, and 'best' now, doesn't necessarily match 'best' in an ideal world.