The alts aren't that much of an issue, until that person also has significant stake and uses it to upvote the alts multiple times a day, while hiding that they are alts.
I agree if the content were too poor for the rewards. Otherwise, it would be a problem to the extent the community did not use their downvotes to trim the rewards.
I'd say that even with significant stake, creating alts just to be able to self-vote more than is deemed appropriate is a losing strategy in terms of rewards. Users with very large stakes tend to be followed and upvoted by others anyway. Posting with many accounts forfeits that advantage.