Having done BJJ for years at several different legit dojos under many different instructors I can attest to the fact that BJJ does not emphasize takedowns at all.
If someone wanted to train one martial art only to be a better fighter, I would absolutely suggest BJJ over other disciplines, by far.
It's very different than other arts. With striking, even though one may not have experience, generally they know the idea is not to get hit by the other guy and try to hit him. Wrestlers showed in the early stages of MMA that they could not finish a fight to save their lives, until they learned to utilize ground and pound. Ground and pound is a discipline in of itself. Definitely not taught in the sport of wrestling.
BJJ is a sport that when you begin you're like "wtf?" It is not exactly intuitive at all at first. Also, I've never done any sport where the progress is so rapid. Within 6 months, I'm tapping all the newbs that walk through the door, even if they're bigger and stronger.
More than that though, BJJ teaches you confidence in every position. What art has you fighting from so many positions. And you learn so much about each position as well. In boxing, you and your opponent are standing, face to face. Wrestling, you're just trying to throw and pin the other guy. But in BJJ you could be fighting from standing, you could be standing over the other guy's guard, kneeling over his guard, half guard, mount, back mount, side control. And of course you could be on the other side of all those positions. Before BJJ, if someone bear hugged me or got me in a headlock or somehow I found them on my back, or mounted me, what would I do? I wouldn't know what to do. But when you fight from those positions hundreds of times, you're very comfortable there. If someone hasn't trained grabs you anywhere from any position, big trouble for them.
Boxing has had success in MMA only when combined with other stuff. How many boxing matches end up in KO before even one clinch. Hardly ever. Well in MMA there's no ref to break you up. What is the boxer going to do when the ref doesn't break the clinch? He doesn't know. He's never trained that before.
What will the wrestler do when he pins the guy, but the guy ties up his arms and the wrestler is not familiar with using his base in such a way that allows him to put power in his strikes? This is an art too. When Dan Severn was on top of people, he would pitty pat strike. Couldn't generate power.
So as a stand alone discipline, BJJ and it's not even close. One must at least have defensive BJJ (much easier to learn than offensive) in order to survive or change positions to utilize their striking. That's enough out of me.