>> A new reddit could be coded in weeks.

Well, it wouldn't take that long, given that the entire Reddit code base is open source: https://github.com/reddit

Really, anybody can literally clone Reddit anytime they want to.

But not only is it likely impossible for a competitor to attract a significant portion of the Reddit user base, what would a competitor do assuming they did manage it? All a competitor would have done is bought themselves a money-losing operation. Unless you're a charitable billionaire or a well-funded non-profit organization, why would anyone want to do that?