Agile Is Not About Predictability
The moment you make Agile about predictability you've already lost.
The whole point is that there will be surprises and you can't predict everything.
You don't need Agile to be predictable, but to be adaptable.
And when you're more adaptable, you will make more progress but don't confuse that with being predictable.
And with this in the back of your mind, why do you have a long product backlog?
If you know all the steps already, what's the point of adapting?
And if you don't know all the steps, why pretend like you do know?
Every step you take helps shape the way. And if that doesn't apply, then you don't need Agile.
I've been through a ton of teams, companies, and projects over the last decade. I've yet to see a team effectively size their work and create in a way that was predictable unless it was super super small bodies of work.
I've only known the teams I've worked with to adapt to the chaos of the day-to-day. I still find it fascinating that there are teams that have their shit together somewhere in the world enough to use predictability.
A thought that occurred to me while reading your insight... Indeed it's not about predictability strictly speaking. But could it be said that in some broader way there's something to it, in this more precise sense: being more predictably successful (as in increasing the chances) in achieving the outcomes we need to achieve by being adaptable and advancing incrementally?!