I will play devil's advocate on this one, not because i think everyone should drop their DSLR, but because no other fully fledged mirrorless user has commented yet.
I will go through the author's reasoning one by one:
1) You're invested in a system: i hear this a lot and yes, it's true, we spend money on our equipment. Though, it's not an investment since, on the most part, we do not get any monetary gains from it. Further more, if you sell your gear, you sell at a loss. Financially speaking, it is not an investment.
2) Not limited by your kit: True enough and fair point. Though, we could always get into the debate about speed and feeds, which we always do and which leads us to buy new gear. Anyhow...
3) Money would be better spent elsewhere: While he has a point, where one spends their money, is their own business. While an "experience" may get your juices flowing, same can be said about getting a different tool to do what you like. Each to their own.
4) you know your system: Again, he is right. We all get to know our specific cameras quite intimately, so why would we want to go away from that comfort zone? Well, for me, trying something new is one of the spices of life. There is so much more in the world then brand X or choice A. Change is good and we are not so stupid to not be able to learn a "new" system. ISO is ISO, Aperture is Aperture, shutter speed is shutter speed, etc. It took me a few days to learn where everything was on my new camera and maybe a month to get comfortable with it.
5) Lighten the load: Yes, this is a big one and has somewhat turned into a cliche. Admittedly, the gear is lighter and less bulkier, simply because it does not have a mirror box. In the end, it really comes down to, "I don't need something that may weigh twice as much, to get the same results."
6) Focus on what you actually need: Well, yes, he is right again. But, honestly, do ALL of us do that? Seriously, for all the people who have FF cameras on this forum, do they REALLY need it? The answer is no, they don't. Each one of us is guilty of getting something we don't really need, but make excuses to justify it.
7) You don't have to keep up with the Jones: It's actually "Joneses". I don't see what this has to do with mirrorless cameras, but i will roll with it. No one can tell me that people don't buy the latest DSLR just because their friend did... gimme a break. This one should have been left out.
8) would you be more excited owning it then using it: Again, this applies to any camera or purchase for that matter. Who doesn't get excited about getting a new gold box? How many times have a read about people waiting on the UPS truck? This has, once again, nothing to do with mirrorless cameras.
What it comes down to, is personal choice. A camera is a personal choice. His reasoning is not very solid in my opinion and leaves many openings for debate. Photography should be fun. It should be enjoyable and if that joy & fun is partially due to you using a DSLR or a mirrorless camera, great. One tool is not for everyone, but i feel that this article is stating otherwise, although in a subtle way. This is just another article telling us to stick with something for half baked reasons and not look beyond a particular silo in the camera market. This is no better then someone telling you to dump your DSLR and go mirrorless.