Lyanna Would Have Been Miserable With Robert
really don't understand the argument that Robert Baratheon would've been a completely different man if Lyanna had lived and married him.
Robert was already Robert before he lost Lyanna. He drank, slept around, and already had three bastards by the time of the Rebellion. None of that was caused by her death.
And people also forget one important thing: Lyanna didn't love Robert. She didn't even believe he would be faithful to her. She literally told Ned that Robert's nature wouldn't change just because they married.
Maybe losing Lyanna made Robert worse over the years, but marrying a woman who didn't love him wasn't magically going to turn him into a faithful, responsible husband.
Lyanna living wouldn't have changed who Robert fundamentally was.