Competition kills competition. To the victor go the spoils. Bigger war chest, bigger army. Etc.
The reason I defend capitalism is not because I like the outcome. I just have good reason to be extremely skeptical of the alternatives (judging by history). Systems that don’t take into account people’s natural competitive instincts are doomed to catastrophe.
For anyone who hates hierarchies and dreams of a flat system (some form of anarchism), I would invite them to read The Tyranny of Structurelessness by feminist Jo Freeman.
Anarchism does not (necessarily) call for a total lack of organisational structure, first and foremost it calls for the abolition of unjust hierarchies. I think a lot of anarchists would broadly agree with the main points of that article.
If you think there is no viable alternative to captitalism, I'd highly recommend the book "Capitalist Realism" by Mark Fisher, which tackles that very subject :)
Capitalism doesn't need defending, even if alternatives are worth critiquing. A system alone will never be enough. No matter what, conscious effort must be taken to prevent suffering and total collapse of the social contract. Efforts to better the public good are more prudent than ever with humanity being so powerful and dangerous.
Survival of the fittest is not a defense of capitalism as a public serving economic system. It's an explanation of how capitalism pushes us towards authoritarianism, then autocracy, then jungle law, where the public are left to the elements and parasites.