Vegans have lower rates of some cancers and higher rates of others. They also have astronomical rates of mental illness. The best studies that we have don’t really show life expectancy differences between a vegan diet and omnivorous diet if you account for healthy lifestyle factors. And no, ultra type athletes are some of the least healthy people on the planet. they have incredibly high rates of things like heart attacks for the record. This isn’t necessarily a knock on veganism in this context or those athletes. when you push any exercise to the extreme it isn’t healthy.
The question being are plants necessary? No. If you don’t eat them I would recommend you get a diverse intake of animal products though. Lots of organs and collagen. If you want to include carbs you have dairy products if those are suitable and things like honey. I have eaten very little in the way of plants in about a year And my performance is great and my health markers are as good as they have ever been.
Human ancestors, especially if you’re of European decent, have gone almost entirely on a meat diet for millions of years until only several thousand years ago. We retain some of our plant eating ability as survival food but our digestive tract and biology are built to eat meat as a primary source of nutrition.