Being in my late 20s now, 1250kcal is just not sustainable, but going with keto is working great and is likely to remain so for a long time. It helps that all I ever drink are water, tea, black coffee, and milk. (That's been the case my entire life, so that part isn't unusual.)
I eat eggs/sausage/bacon for breakfast (one of the three), then I eat an earlyish dinner of steak or bratwurst. Since I'm on Adderall XR, lunch never happens. On days I feel like carbs, I go out to Ivanna Cone (amazing local gourmet ice cream) for half a scoop. That consistently puts me at less than 30g of non-fiber carbs per day, with a fairly steady decline in overall weight. I'm never hungry outside of meal time, and I don't have problems saying no to carbs, aside from when I'm eating out with friends.
Once I've lost the weight I want to lose (another 20 pounds or so) I'll be able to figure out what level of carbs I can have without gaining on a daily basis, and live with that.
I have to eat carefully, but not eating carefully is what got me fat in the first place, so that's just what I'll have to deal with.
If you are 5kg overweight, a "sustainable lifestyle change" will be sufficient. And there are those people for who even the severe risks of a gastric bypass outweigh the damage their weight does. Quite obviously, in between those extremes, there will be people were an unsustainable diet will be healthier than staying at their weight or losing it only slowly. And why not choose a diet that has been shown to minimize the loss of lean body mass.