Losing weight is hard. Maintaining the loss is hard too, but not unsustainable. It might be deemed unsustainable in a large meta-analysis simply because a majority of people don't discover a solution that works for them. But that conclusion is purely mathematical and academic. Individual success has nothing to do with large sample statistical data (unless, possibly, if you are highly susceptible to the sucking-vortex of the herd-mentality).
I tried the exercise and (reduced calorie) diet route for years (the herd!). I was in great shape, riding 1000 miles a month, but always carried about 20 lbs more than I wanted (which sucks when you really enjoy hill climbs on a bike). I finally discovered the (LCHF, NSNG, Paleo, Atkins, Primal, WAP diets). So many different names for essentially what amounts to a whole-foods diet. I lost 30 lbs in about 6 months. Stopped bike commuting (due to birth of daughter) and started walking. It's been 18 month now. Weight is still off. Waist has gone from 38 to 31. Pre-diabetes has reversed. Several other health markers have only improved. Cardiac scan show zero signs of heart disease (at age 47). Oh, and cholesterol...forget about it. I've always had normal total cholesterol levels, but it's also always been pattern-b ("too many" small LDL particles). And as we all "know" pattern-b will "give" you heart disease. Well, the scientific jury is still out on this one (although the press jury, and government-jury have reached their decisions). So I have pattern-b cholesterol, but no other indicators or risk factors for heart disease. Cholesterol doesn't matter, at least for me.
The diet has morphed into a lifestyle now (i.e. I don't have to "work" at it any longer, its just there). I don't count calories. I feel better than I have in years.
NSNG is the best term I've stumbled across that describes the diet. No (added) Sugar, No Grains. Everything else is game. Meats, yes! Eggs, lots! Cheese, you bet! Nuts, love 'em. Minimal, but some, non-sugary fruits. Lots of veg. I hate the term "Everything in moderation", but there are exceptions, like sushi rice (which (may) have it's own benefits. Google "resistant starch").
We'll see where this goes in 5 to 10 years. I've seen what the SAD can do to a susceptible person. My mother, who's became diabetic (strong family history) in 1985 has no kidney function (dialysis 5 days a week), and no left leg, no strength, no mobility, and a host of other medical condition. I'm not going down that road if I can help it...meta-analyses be damned. Seeing that can be very motivating.
The thing to keep in mind is that by the time a scientific study makes it out of the science community and into the press and blogosphere it has been massaged (and perverted) so much as to be almost meaningless.
A great book on the subject of interpreting health related studies and data: Death By Food Pyramid This is not a diet book, it is a history book. Well worth a read.