There is no doubt about it. Weeds are sustainable. Otherwise, they would not be weeds. By definition, they grow where they are undesirable. Less sustainable vegetation should be less invasive. Also, it should be less resistant to eradication than most familiar weeds are. Unfortunately, also by definition, weeds are undesirable. They can not become fads.
Sustainable horticulture is a fad though. Unlike most fads, it is actually quite sensible. In theory, it is horticulture that requires as minimal intervention as possible. It excludes that which requires intensive or impractical cultivation. For example, native species that grow wild are sustainable. Tropical species that may survive only within greenhouses are not.
A problem with the sustainability fad is its marketability. ‘Sustainable’ and ‘Sustainability’ have become cliche buzzwords. They too often describe merchandise that is contrary to the fad. Realistically, genuine sustainability is unsustainable within profitable marketing. Truly sustainable merchandise would eliminate most of the need to ever purchase more.
Modern cultivars can qualify as ‘new and improved’ as they first become available. They are certainly new. However, their improvements may be questionable. Hybridization and extensive breeding can cause genetic deficiency. Even natural variegation compromises vigor. Seed is not true to type. Most aesthetic improvements are contrary to sustainability.
Native species are technically sustainable. Once established, they might survive without irrigation or other attention. Unfortunately though, some are not very adaptable to refined home gardens. Some are vulnerable to rot if nearby vegetation needs frequent irrigation. Some perform vigorously only for a few years. Several species are innately combustible.
Ironically, several of the most passe and old fashioned species are the most sustainable. That is why some of them became passe. Lily of the Nile can survive indefinitely. If it gets overgrown, it is easy to divide and relocate. It may be available for free from neighbors or friends. African iris, New Zealand flax, bergenia, most aloe and many yucca are similarly sustainable.
Tony Tomeo 
Sometimes I think we should like weeds. As a friend said, we pull them here 365 days. Native and wildflowers are popular to grow as they are very hardy.
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Actually, I do enjoy some weeds. If I could live in town, where they would be less likely to escape cultivation, I would enjoy more. I want to grow Cortaderia jubata, which is the nastiest and most aggressively invasive of the pampas grass, merely because I remember it growing wild on the coast of San Mateo County when I was a kid. Also, I would like to grow Acacia dealbata because I remember it from college. There is an overgrown canned specimen of Eucalyptus globulus in the garden right now, just because I like the foliar aroma. I have no idea of what to do with it, and will likely need to discard it as it gets too big for containment. Even if I could plant it in the ground, it is too disfigured from getting pollarded. Also, there are a few #1 cans of Arundo donax. I can actually grow those like potted bamboo, and just be very careful with how I dispose of the debris when I cut them down every winter.
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Sustainable horticulture is just another fad because it is the gardener and the climate that require changes. Most of us get restless and want to grow something new and different. Or we lose plants to weather extremes.
Solidago is sustainable in my yard, but it is a total thug and crowds out other, more desirable plants. And its season of interest is very late. I weed out such sustainable native plants to maintain a ‘garden ‘ rather than a weedlot. Most of us want to cultivate beauty when it comes down to the truth. Beauty usually needs some maintenance and care.
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Yes; sustainability has potential to be a practical fad, but that would not be profitable enough. Natives should be most sustainable in most climates, but for us, some enhance combustibility and are not as visually appealing as exotics. Some exotics are a good compromise, but some need irrigation to be appealing. In my garden, lily of the Nile is very sustainable, and really can survive without irrigation. (Some of ours gets none.) However, I prefer to irrigate it somewhat. I am more satisfied with that compromise than other species that pass for sustainable, like so many bedding plants.
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Trees and shrubs tend to be the most sustainable of all- and many ferns. We prune them mostly for our own purposes, though it can help their health and give them a better form and longer life.
Sustainability is a good theory and a worthy goal. It takes a deep knowledge of plants, and of your own environment, to make it practical, in my opinion. Like your understanding of how Lily of the Nile performs in your gardens. That is real knowledge of place and species, and must be the basis of any real sustainability.
So many plants said to be sustainable in our area- like Echinacea- tend to be short lived and to need trimming back. My best white Echinaceas were grazed by deer this spring. Even heirloom Iris require irrigation, trimming, and occasional feeding to do their best.
It takes years to really understand which plants will be sustainable, with the fewest possible inputs, in a particular setting.
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It is easier to identify what is not sustainable. Annual bedding plants are certainly not sustainable if they are not allowed to go to seed and naturalize. Yet, they are commonly marketed as such.
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