For my module on Life Cycle Thinking and Circular Economy with Angela Druckman, one of my classmates asked something akin to the above question on the first day, and it happened to be one of the questions our professor wanted us to be able to answer by the end of the module.
Circular Economy (CE) has been one of those buzz words in the sustainability space over the past few years, along with things like life cycle assessment, zero waste, sharing economy and so on.
But what does it really mean?
It challenges the traditional take, make, and dispose systems prevailing in society that Annie Leonard talks about in The Story of Stuff.  In a circular system, the goal is to design waste and pollution out of the system, and keep products and materials in use, ideally in a closed loop from sourcing, production, distribution, use reuse, refurbishment, recycle. 
By designing with circularity in mind, companies can sometimes get a more reliable source of raw materials (i.e. if they successfully recover materials from consumers locally vs. importing virgin material), and divert waste from landfills.
How can a company make this happen?
One of the guest lecturers in our class came from an international electronics company. He claims that the company is diverting 1 million plastic bottles from landfills every day, combined with a 43%, 63%, and 38% reduction in carbon emissions, energy and water use respectively, from using recycled vs. virgin plastic. 93-95% reuse for products returned through its repair and remarketing program. He also highlighted access vs. ownership, in their product-as-a-service, where consumers don’t own a product (i.e. printers or computing systems), but merely lease it from the company (similar to other sharing services like Airbnb, Uber, Spotify, etc.). Lastly, he also mentioned their emphasis on creating modular or easily repairable and upgradable models, complete with repair instructions, which help avoid rendering the product obsolete after such a brief period.
Sounds great! Why aren't more people into it?
These processes can definitely help encourage companies to design more sustainable, modular, and longer-lasting products. However, the R&D that goes into such experiments isn't easy, and one would also have to run a number of tests to see if it is: 1) technically feasible in production / operations, and 2) if consumers / clients will actually want to avail of those products / services.
Furthermore, being green is not black and white.  Sometimes, in an effort to “close the loop”, or avoid creating waste, there may be other impacts like increased carbon emissions, water consumption or pollution, social / health and safety issues (one reason why Human Nature says they can't do refills), that aren’t factored in by simply looking at circular economy. Through my MSc thus far, I have noticed the running trend of “it depends” being the answer to many of the sustainable development questions we have come across.
"It depends" is not a very satisfying response. Surely it would be good for certain applications?
Yeah, "it depends" is definitely frustrating, but also oddly reassuring. The applications and benefits of circular economy are very context specific. In the case of "closed loop recycling", where nothing gets down-cycled into another system, one would need a huge volume of the "waste" product to optimize the recycling process. Materials, components and products (MCPs), must be properly recovered and redistributed for reuse to maximize their remaining functionality within the same system. 
Furthermore, we need to assess if a closed loop is more optimal vs. open loop, wherein the material is no longer utilized for its original purpose. Closed loop recycling can only have a low carbon footprint and overall environmental impact if recycling is done locally.
Take the case of glass vs. plastic, it would only be optimal if the glass is recycled within 300 kilometers of where the waste is generated.  Additionally, in my previous experience consulting for a social enterprise in the beverage industry, we found that delivering glass bottles to customers required so much extra packaging, especially when going through third party couriers or shipping, that it seems to defeat the purpose of using glass in the first place. So, in this case, even if we get to recycle the glass, it still requires energy, water, extra packaging, manpower and other resource input at various stages.
Okay...so are you saying single-use plastic is better?
I'm saying: "it depends". :P While it is of value to pursue circularity, it is also important to look at things in their unique context, factoring in the other elements that make up the complete picture.
As much as I would like to see the end to all single-use plastic, it's hard to deny its viability as a material that makes it easier to safely pack and preserve food products, get commodities to consumers with minimal weight and breakage, etc.
At the end of the day, I simply wish to challenge the default assumption that glass trumps plastic all the time. And that we should constantly ask ourselves what components, processes, and infrastructure (not merely materials) could come into play to make the introduction of circular systems truly a more sustainable choice.
Ellenmacarthurfoundation.org. (2018). What is a Circular Economy? | Ellen MacArthur Foundation.
Muren, D. (2009) Green's Not Black and White:The Balanced Guide to Making Eco Decisions. Barron's Educational Series.
Hahladakis, J. and Iacovidou, E. (2018). Closing the loop on plastic packaging materials: What is quality and how does it affect their circularity? Science of The Total Environment, 630, pp.1394-1400.
Designing a Circular Economy. (2016). The Parliamentary Office of Science and Technology.