No shit. What gave it away, the two terms or the inequality? :)

Joking aside, it's simplistic because it's elementary. If real returns exceed real growth, ceteris paribus, you have a net flow of principal (so to speak) from labour to capital. That doesn't mean one can conclude the argument with those two variables alone. But it's a valid starting point, and concludes with many solutions other than increasing taxes to reduce r.