Sure, but one person not eating beef doesn't have a direct impact on beef farming. People are already eating less beef now because prices have become unrealistic for most people, and yet it's not having a direct effect on the environment. Cattle herds in America are at a 30 year low, but it's mostly small ranchers going out of business. Factory farming is still destroying the climate, the conglomerates are just relocating their herds to better hide the damage and reduce overhead costs.
Personal choices are good. People should make good personal choices, because it is what they believe and how they want to affect the world. It is healthier and cheaper to eat less beef. It's also cruel and disgusting the way factory farms operate, and the processing practices will probably make a lot of people sick.
But your argument that people should eat less beef because it will help the environment is flawed and hollow. Cattle farming is bad for the environment, but it is not true that eating less of it will help the environment. Oligarchs are clearing the rainforests because we've stripped the carbon from American topsoil, and it's now cheaper to import beef than it is to raise cows in the Midwest. That additional damage to what should be protected forests, not to mention the global shipping and transportation, more than offsets the reduction in the total number of cows. And it's not like pig farming or chicken farming or fishing is much better for the environment.
True change cannot be acheived with personal choices. We need legislative action, and we need to demand it of our politicians. It needs to happen now, and it needs to be drastic. We need strict environmental regulations and strict enforcement. And none of that is likely, so you may as well enjoy your cheeseburger.