On the first order, or to the first degree, what you eat doesn't matter. Being a healthy weight matters, which means eating only enough to maintain that healthy weight and not more. If you are a healthy weight, you can eat anything you damn well please and achieve this first-order level of success. And if a pure-meat diet gets you to a healthy weight wereas no other system worked for you, then an all-meat diet is a reasonable way for you to lose weight. Other than keto's effect on hunger, food choices are mostly irrelevant to weight loss. If you eat extra protein your body turns it into fat... it doesn't go onto your muscles. If you eat extra sugar your body turns that into fat too (but you already knew that).

But on the second order, or to the second degree, people at a healthy weight who chose to eat a significant amount of meat throughout their lives don't live as long as people at a healthy weight who eat little to no meat. The healthy-weight people who live the longest are the pescatarians who are lacto-ovo-vegetarians except that they eat fish. They eat no red meat, no pork, no chicken, only sea-meats and generally only the fishes.

The difference occurs due to atherosclerosis. Meat (not just the saturated fat) and saturated fats (like coocnut oil) accelerate the development of atherosclerotic plaques. This is why you will see very healthy weight body-builders on the carnavore diet kick the bucket at 57 from massive heart attacks, and why people like Ancel Keys and nutritionists that worked with him (Jeremiah Stamler, Henry Blackburn) live to 100.