Just because you are not currently a moral realist, doesn't mean there's no such thing as moral realism. Case in point: http://fas-philosophy.rutgers.edu/chang/Papers/Railton-Moral...
>I'm talking about capitalism as a system
And here we've reached the heart of the disagreement: I don't think you are. You're talking about an imagined capitalist utopia. It no more resembles real capitalism (that is to say, the common features and behaviors we can generalize from all actually-existing capitalist systems) than "From each according to his ability, to each according to his needs" resembles Soviet Russia.
>I think it makes sense to start with an ostensibly good system than any other without very good reasons not to.
Why should you try to solve an optimization problem by examining only one proposed solution, and then simply apologizing that all real-world systems with the same name as the proposed theoretical solution are terrible, while not even examining the degree of resemblance between the proposal and the actuality, and never examining alternative hypotheses?
After all, it's not like anti-capitalists lack for proposed utopias (see list of citations at the bottom). In fact, our proposals nowadays tend to have worked in experiments far better than the proposal of "Implement the capitalist utopia of Austrianism/libertarianism" has actually worked in experiment -- starting with the triumphs of the social-democratic era and going from there!
http://www.nybooks.com/articles/archives/2010/apr/29/ill-far...
https://www.jacobinmag.com/2012/12/the-red-and-the-black/
http://grist.org/article/upgrading-capitalisms-operating-sys...
http://www.cornellpress.cornell.edu/book/?GCOI=8014010023835...
And this is all, of course, before we start addressing techno-utopian or techno-socialist proposals that run off the basic concept of, "Well, resources aren't that scarce..."