I like the aesthetic of its usage in this case and find it makes the sentence read easier. It eliminates even temporary ambiguity about part-of-speech for the final two words. It stands in for a clarifying word like "become".
To the extent it's "incorrect", it's in that it generated this discussion at all.
I completely disagree with you and find the comma misplaced in a jarring way. It interrupts the flow of thought for me in a negative way: much more than a brief pause, it places a marker that the syntax of "richer" isn't fully bound to the previous words. There's also no ambiguity in the last two words without the comma.
I think if the author wanted a "pause the sound while keeping the syntax flowing" mark, the ellipsis (...) would have done the trick much better. In my opinion, though, this sentence did not merit any pause between "rich" and "richer" since there's nothing surprising about that word.
To the extent it's "incorrect", it's in that it generated this discussion at all.