Why can’t it be both?
A structured scenario gives much needed purpose so a solo session just doesn’t meander off a cliff. Most of my angst with solo rpg-like quests is my mercurial motivation. It takes a lot of energy to keep finding “the reason”. Having a more structured quest-line gives you a goal and a finish line to aim for.
That said, emergent narratives always seem the best. It’s great when something unexpected happens along the way. So give me an over arching quest line with a unique path between acts.
Dark Venture goes quite a ways with this concept by providing a fixed time frame, and random side-quests nudging you along the journey. It’s just a bit fiddly, forcing you to delve too often into the card decks.