We know in Scrum, the responsible for make estimations is solely responsibility of the development team, but in long term planning, for example a release planning, we'd to have some kind of estimation about all of PBI in order to feed planning burn up or estimates the amount of sprints needed to complete the product.
So is resonsible the Product Owner to make this estimations, and how the PO could perform this in order to make a realistic long term planning?
The only people who an accurately estimate is the people doing the work. The product owner doesn't generally code and even when they do, their job is to ask for the team to do as much as possible. The team's job is to set their limit and say no.
I work with enterprise organizations and help them with longer term forecasting. The first thing is to move away from story points and look at cycle time. Google Troy Magennis and focusedobjective.com/. He's got a tool for doing cycle time.
If I can't get an organization to measure cycle time, then I use past work estimates. I have them look at the work they've done in previous releases and use relative estimating (not planning poker) to create estimates on their old work. Then I use this as a tool for estimating future work. You can use this method, easily, for larger items (features, epics, etc.) as it doesn't require a lot of data, just past reference of "done" work.
Just like your short-term planning, the team is asked to make any long-term estimates as well. If you are using story points, they can account for unknowns and risks as well as effort, so the team should be able to make long-term estimates without all of the details.
No, individuals doing the work decide on deadlines for the individual items in the backlog, but the product owner ultimately decides what trade-offs are needed in order to meet the deadlines required for the project.
For example, if the product owner finds that launching the product in two months is absolutely needed because they want to get ahead of the competition, but the plan accounts for 2.5 months, she will have to remove some features from the backlog or find ways to get the job done in 2 months, obviously negotiating everything with the development team.
