As a counterpoint to the idea that Leia’s vision showed her son would die, remember, a major motivator for her in TFA was the idea they’d “lost” their son. So just like ‘Vader killed Anakin’, we can interpret that ‘Kylo killed Ben’, and that was why they called him lost. His fall was worse than his death, so I think the vision (which, yes, included his death) doesn’t need to be a prophecy which was ultimately fulfilled, because her main fear there, just like Luke’s major regret, was his fall to the darkside. His redemption is what she’s after.