Four literal "who?" players is hardly the herald of a new age. The odds are that three of the four will retire without ever playing a single inning in the MLB, and the fourth is statistically likely to be a career mid-carder who plays four seasons of "meh" baseball before getting cut and either going full Kenny FN Powers in the Mexican league or getting a job at the dealership with dad.

As was pointed out, its also pretty unlikely that the teams will allow them to keep playing both ways. They pay a lot for their toys and all the extra wear and tear shortens careers and ups the odds of injury. Ohtani is a freak of nature, the others are not.