My experiences disagree with that; a group of specialists is far more common in D&D than a group of generalists. The most common classes I've seen in 5E are barbarian, sorcerer, and bard, and only the bard is actually versatile. The barbarian and sorcerer are powerhouses that only do a couple of things, and do them very, very well. The bard is an exception, but it is a versatile class that has access to healing magic, useful to support another healer or to serve as the healer with other uses. The most versatile class in the game, the wizard, is seldom anywhere to be seen.