How naive to present your argument like this. This is merely not about "100% with them or against them" kind of thing. If you prefer capitalism and would rather seek reforms to make the capitalist status quo "more humane", it simply means that you do not see capitalism as an inherently exploitative system because you inadvertently support and justify its existence. Ultimately, anyone who supports capitalism and the status quo are *against* them (by them obviously means the leftists who oppose capitalism as a system). This is common sense.
"Economical natural selection" way to out yourself as some brainlet, then. AOC is not even leftist, she is a social democrat who intends to model her system after the Nordic model, which is still inherently capitalist and relies on the exploitation of others to make capital. The Democratic Party in the USA supports this, no matter how much lip service they do about "taking from the 1%, not supporting lobbies, etc." What the Dems propose in the US is nothing radical at all by any means, if you go to Europe you'll see that this is a pretty centre position.
At the end of the day, the term "leftist" means very little. It is either you are truly on the side of the oppressed working class, or on the status quo that preserves capitalism.