... you will never turn back
Why should we estimate? What are we considering when we do it? Yes, estimation accuracy is a matter of practice maintaining the same team constant. What and how can we estimate? We can do it all over hierarchy of requirements, since strategic levels until implementation ones. But essentially a good product planning should be done to slicing features maintaining product value and introducing complexity across the product development phases.
When Agile Transformation enters in a scale process, the challenge starts. It is mandatory regular alignment across the different portfolio levels to make sure that the Transformation occurs.