Christianity was already a major religion and was on the up, so I'm inclined to think that sooner or later you'd inevitably get a Christian Emperor, unless the Empire itself falls before this can happen.
But which Christian Emperor?
A different Constantine the great?
Or maybe a Julian who dies after 2 years and the reaction was "well that was..... interesting".A Christian Commodus who leaves the public going "well we're never doing THAT again."
I don't think the Tetrarchs would have abandoned their persecution if they thought that were the case.
The history suggests that they didn't understand what they were up against. Christianity was not something they were used to.
Christianity was expanding. But the victory they got otl was not inevitable.