[alabastervlog]

There aren't fixed rules, even to the degree to which there are such rules for other grammatical questions in English. Much of comma usage comes down to preference.

I think part of why we've shifted so strongly against their use is because if you leave it up to taste, as had previously been common, most people make poor choices.

It's funny because even as we've moved away from prescriptivism, the "rules" around comma usage have tightened and people have gotten quicker to call a given previously-common usage incorrect.

[pclmulqdq]

There are rules around comma usage that are up to taste (eg the Oxford comma), and people get very dogmatic about these rules for bad reasons. There are also times when usage of a comma is incorrect according to all the known rulesets for English grammar. This usage is in the latter category.

The notion that a comma is any sort of pause fell out of favor in written English in the 1800's and thankfully hasn't been back (see the second amendment as to why "a comma is a generic pause" is a bad idea). You would have to be the loosest form of descriptivist to say that this usage is close to correct, and I would question whether you would accept any grammar rule at all at that point. Many people use run-on sentences and many don't capitalize the start of sentences in very casual text, even though these are widely (universally) accepted rules.

[alabastervlog]

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.

[pclmulqdq]

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.