As per the DTI books, alternate universes can be created, but they need a huge change from the “prime” timeline to stop their waveforms collapsing back into it. I.e, the human physiology difference Discovery added the MU. That universe was unfixably changed as soon as that mutation took root. The Kelvin verse too, the destruction of the Kelvin hugely altered the life of one of Starfleet’s most famous officers and saw the destruction of one of its founding planets.