I actually have a different take on what the "basic" problem is, having done a fair amount of research (reading a
lot of biomed research & studies over the past few years) and personally effecting (and more or less maintaining) a huge improvement in my own metabolic health. [1]
To me, it seems obvious that sustaining a low calorie diet (LCD) long-term is basically impossible (from a maximal fat oxidation and, even a thermodynamics perspective). On the bio-mechanistic side, it's also important to note that hyperinsulemia inhibits lipolysis [2], which is one factor that is also usually not taken into consideration. Anyway, while a LCD may be effective for a while (maybe even years), it's probably quite miserable and almost by definition, can't be a sustainable lifestyle change. Even as a short term fix, it may not a very healthy and might be counter-productive long term... [3]
To me, the biggest problem is that T2D is in many (but, to be clear, not all) cases, the end stage of a metabolic patho-physiology that can be identified by other markers (as simple as WtHR, or cheap labs like fasting insulin, or NAFLD indices) that are cheap and easy to measure, but from my experience, most clinicians are completely unaware of this. Which is too bad, since the overwhelming majority of their patients in the US these days (over 80%) are suffering from metabolic diseases. [4]
Over the years (I moved a lot) I've had close to a dozen different primary care providers, and diligently did my annual health checks, all while slowly putting a couple pounds each year and none of them were able to give any useful treatment or even medical advice as my MetS markers crept up. I had to figure it out myself - in fact most gave bad advice and weren't even measuring or diagnosing the right markers. The real kicker to me is how cheap some of these tests are (fasting insulin is I'll also mention that both from the literature [5] and my personal experience [1], some of the worst damage can be reversed in a matter of weeks. I did labs right before I started my diet changes and 1-month in, and even in a single month my TGs and my liver numbers moved from unhealthy to healthy ranges.
As for sustainability, there are some lifestyle treatments like Virta's that have shown better than medication retention rates and efficacy, so it's possible to do... https://www.virtahealth.com/outcomes
How do they do it? A combination of responsive/actionable biomarker monitoring/feedback coupled long-term coaching and peer-support groups. Not impossible and way cheaper (and better quality of life!) than any pharmaceutical alternatives.
[1] https://randomfoo.net/2019/09/17/1-year-personal-health-inte...
[2] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3753874/
[3] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4989512/
[4] https://www.liebertpub.com/doi/10.1089/met.2018.0105
[5] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5813289/