Animals don´t have dreams

For context: I'm not vegan. Yet, I know veganism has, to a broader scale, the best arguments. I don't agree with it too much on the ethical side, but I know its the best option regarding environment, climate change and, why not, to give the animals a better treatment.

Now, to my argument: I've read on different online places an argument that cows (to put an example) are killed at an age that's analogous to kill a human at 8 years old or so (considering the animals lives in captivity, cause in nature they would die way younger in average). But my question is, if an animal is given a good life, and then is killed without pain, fast, unnoticeably, does it really matter we kill them young? It's not like they're going to do something with their lives, specially livestock that has little ecological role in most parts of the world (actually invasive in most of it). They don't have dreams, projects, achievements, a spiritual journey, a career, something to look forward to.