Noam Chomsky, with his two-ish fields, springs to mind. Asimov and Bertrand Russel might qualify.
It seems, though, that those sparse matrices are largely populated by creative efforts; we see many professors of hard science who are also philosophers, historians or writers, but we seem to have lost the biologist, engineer and composer of da Vinci's ilk.
I disagree with the metaphor. 'Sparser matrices' implies homogeneity of effort and accomplishment. It's closer to 'sparser trees' where genuinely regaled accomplishments happen in leafs quite far from the root.
Polymaths may become something more of the lucky geniuses who have the adroitness to envision great trees, the tenacity to grow them, and the luck to not waste too much effort in directions they cannot see to fulfillment. They build branches wide and strong with the right root structure and a strong trunk. Other grow bushes or tall, skinny trees, perhaps easily blown down.
It seems, though, that those sparse matrices are largely populated by creative efforts; we see many professors of hard science who are also philosophers, historians or writers, but we seem to have lost the biologist, engineer and composer of da Vinci's ilk.