I agree that you will see a lot of guys that have strictly MMA training.
But I think the majority of top guys will always be guys that have a strong background in either striking or grappling and have become exceptional at it.
I have to disagree simply because of what I see coming up in the gyms and clubs now, of what we and other coaches are training. They aren't coming to us from other styles very much now, we do get older students who do but not the young ones. Youngsters are seeing MMA fighters and are going straight to MMA training bypassing single styles. It's the future, for now and I daresay quite a few years to come the best will be those who have a strong base style but watch the youngsters coming up.