I agree but with a caveat, or another consideration, if you will.
Those named are great examples but I believe they are outliers. If you took the bell curve of karate practitioners, and put them in MMA fights I do not believe they would fare well. If you took the bell curve of boxers, and wrestlers and put them in MMA fights I believe they would fare far better.
Now, I have no data to support this because it has never been done. However, based on my observations, karate as a whole appears to prepare you the least for a full contact confrontation in terms of the skillset it gives you and the way it is practiced.
I think Karate would have to make the biggest adjustment in order to transition, because you would have to supplement your training with probably much harder conditioning, take consideration to weight classes, and become accustomed to full contact as opposed to point sparring. Boxing, and wrestling already have tough conditioning by nature, have to make weight classes, AND are accustomed to full contact competition with the intent to stop the opponent as the primary objective.
I know people will point to Kyokushin, however, that is not the most popular style in the world. The most popular styles revolve around point fight kumite. Boxing, and wrestling again are the same wherever you go in terms of application, and intent. Without great modification I think coming with pure karate as your base it would leave you the most unprepared, and you'd have to make the most adjustments.
Here I would say that's probably based on opinion more than anything.
Having trained Karate, Muay Thai and Boxing, as well as some grappling on the side for fun I'll give you my view on what you just said.
Most Karate practitioners across all styles are hobbyists with no intention of competing. That's also quite true in Boxing in Muay Thai but at least the sport itself is based on competing and fighting, so you'll most likely have a higher number of competitors. On top of that, you also have the Karateka's who don't believe in fighting and don't think you should be fighting at all.
Even if we take this into consideration though, I believe Karate as a martial art would give you more tools than Boxing to transition to MMA. First of all a lot of Karate is done without boxing gloves, usually the sparring and competitions are with small gloves, like mitts, similar to MMA gloves, so that gives you a better adaption on that aspect. On top of that you also train a lot of other strikes than just punches such as kicks, knees, etc. and from time to time some form of grappling with Jujutsu locks and some kind of Judo type throws too. The stance is also a lot more suited for MMA with weight on the front leg and the in and out type of movement than the boxing stance which would get your legs kicked to shreds or have you get taken down. Based on the variety of techniques taught, the type of gloves used, the grappling being taught, the stance, etc. I actually think Karate would work better than straight up boxing from which you'd have to change your stance, start from scratch in grappling, and learn all the other strikes which aren't just punches. For someone who never kicked or grappled, there's quite a lot to catch-up on already. The full-contact adjustment usually doesn't take as long to get used to (I know what I'm talking about as I cross-trained from semi-contact point-scoring Karate to full contact).
The full contact aspect is definitely a plus though as indeed for people who don't practice full contact it will change the dynamic quite a lot when switching to MMA. While there are many semi-contact Karate styles there are also a lot of Karate styles (not just Kyokushin) which are full contact.
I have a hard timing trying to name someone from boxing who did great in MMA, of course you will probably say it's because there's more money in Boxing so they stick to it, but generally the few boxers I've seen having a go at MMA or even Kickboxing usually had a pretty rough time and didn't stick to it.
There are also weight classes in Karate by the way.
Anyway, my response was to someone who said "Karate is an inefficient fight in MMA", and I think that's far from the truth. It needs some adaptations and cross-training sure, but that's the same with boxing, wrestling and anything else; it has to be adapted for MMA and some cross-training needs to be done.