As with the Betamax vs VHS format war in the 1980s, cost and installed base ultimately determined the winner.SDexpress will be as fast if not faster than most CFexpress cards. And more common
High end cameras have changed memory formats relatively frequently (CF, Cfast, XQD, CFe A/B and UHS-i/ii) depending on their need for speed.
If there is no/limited cost difference for the card slot to handle multiple standards then the camera manufacturers won't have an issue. It is really about what the market is for the cards themselves. If the volume for SD Express is not high then the price won't fall.