When the Warriors arrived in 1971, Oakland had teams in all four major sports: the N.F.L., the N.B.A., the N.H.L. and Major League Baseball. Only six other American cities could make that boast at the time.

Now it is a place to flee. The Raiders, the A’s and the Warriors are all looking to get out, seeing greener grass beyond their concrete-and-blacktop complex.

It is possible, if not likely, that the Oakland of 2020 will have no major teams.

The Warriors have plans to move back to San Francisco, to a glassy waterfront arena, as early as 2018. The Raiders and the A’s, trying to disconnect from each other and from the decaying stadium they share, are threatening to leave, too — the Raiders back to Los Angeles, the A’s to San Jose or anywhere else that welcomes them to a better ballpark.