Long-time illustrator here who has worked for various clients such as WIRED, Slack, etc etc. Here are my two rambly cents:
Firstly, I think this article is almost spot on. One of the major culprits behind this issue of "flat illustration" as I've seen it called is that there is usually a lack of originality being forced onto the illustrator from the hiring companies. Usually this is in the form of "we love what Company X is doing and would love to have something visually similar," and by visually similar, usually what they want is something that's pretty much the same but with a different colour palette. Boo!
But hey, if you're the illustrator here and they're not going to credit your name (which is generally the case with branding related work, such as this flat stuff) ... who gives a shit, right? Take the money and run and watch the world burn with flat illustration. /s
I said "almost spot on" earlier because one thing not mentioned in this article––and this has been my "theory" and those of my illustrator colleagues for years-–is the economic side of it all, and how it has affected illustrative output:
Firstly, know that illustration rates have been pretty stagnant since the 90s and deadlines are getting shorter and shorter due to the demands of real-time publishing. No more waiting for something to go to print, you just schedule a blog post.
Back in the dying era of print (mid-00s), even small illustration gigs could come with generous turnaround times of 3 or 4-ish weeks if you were lucky. Usually a week or two to figure out rough sketches and get approval from the client, and then a week or so to crank out the final art. Maybe you're painting or drawing this on physical media like paper, gouache, watercolour, etc, which comes with fantastic built-in features like texture and brushstrokes, where honestly flat is HARD to pull off.
But today it's more likely you get an email and the client wants sketches for Friday, and final art the following Wednesday. You simply don't have as much time to work on polishing up a nice sketch to turn into a painting; sometimes this stuff just needs straight-up time to massage into place. Instead, you can use the old bucket fill in Photoshop or flat fill in Illustrator, copy paste a few pieces from your own collection of past work perhaps, adjust a few colors and call it a day.
I admit when I first saw this stuff 10 or so years ago, I thought it was bold and graphic and honestly I wouldn't have had the courage to do something so simple; it's a nice shout out to the digital painterliness that is flat vector shapes. But when it becomes this common place ... well, it just looks boring.
Anyway, as much fun as it is to dump time and energy into a drawing, if the requested content is fucking lame and generic and the deadline is short, have fun getting nice original work out of it. Instead, maybe hire an illustrator on the long term and have them develop something truly unique that no one would want to be caught dead copying because it would just look like plagiarism.
It's not all doom and gloom though. Some of this flat stuff can be good! For example, many illustrations on the Apple App Store (on macOS, depending on the day) have a nice level of polish to them that push them past the generic flat style and can feel fresh and new, at least to me as an illustrator, while still satisfying some obvious corporate requests for something safe and predictable.
*edit*: spelings and puncs
*edit edit*: Corporate Memphis is such a dumb name for this. just call it Flat Style as all of the illustratorverse has been calling it for years.