The collective knowledge of Newton, Einstein, Lagrange, Maxwell, Bohr, and hundreds of other physicists are written down and more or less distilled into the brains of thousands of physics students within a decade. We already have a never ending cascade of innovation and progress. Getting new perspectives and fresh minds on the problem are more valuable than wringing another year of work out of the aging geniuses of yesterday, which is why many of the greatest physicists like Hawking and Feynman were so focused on teaching and popularizing the field. Yeah, it would be great to still have Newton around, but it turned out to be even more useful to have Lagrange around instead.

Besides, if you look at the actual lives of these people, most of them stopped producing useful output eventually. Newton spent more of his life arguing about theology than inventing calculus and physics.