YDS: Who Attends Product Backlog Refinement in Scrum?

How is that even a problem considering a full scrum team is made of 1 PO + 1 SM + 5-9 Devs?
The PO needs to be there, so that one is Must Have. The SM needs to be there, so Must Have. Then, how up to 9 people is it a problem for having bandwidth for one of the most important ceremonies of Scrum allowing to understand the product vision, direction, discuss what and why is accepted for development, where priorities are discussed? Also, refinement takes only up to 10% of sprint time. If devs don't have time for this meeting maybe the sustainable pace of dev is already compromised. This refinement meeting prepares sprint planning, so it is crucial to guaranteeing quality. I don't see how it is compulsory for the dev team!

Also, nothing prevents PO to have other meetings with stakeholders to understand requirements and have longer conversations about strategy, vision and epics to then bring that to refinement meetings with the dev team and present those findings in the form of either new stories or refinement of previous stories, saving longer conversations. But having all dev team there (again 5 to 9 people) seems crucial to me.

Communication is at the heart of Agile. Quality is at the heart of Agile. Sustainable pace is at the heart of Agile. By missing this meeting you're lowering those: communication, quality and sustainability of Scrum.

Hi all! I'm Barbara, this is my first post and I beg you pardon for my bad english...
I've been in 2 agile prj. In both of them all the members of the scrum team attend the refinement.
Scrum tells that, even if a single story is developed by one member, all the dev team is responsible for the story.
How could it be if not all the members attend the refinement and perfectly understand the PO desire?
TY
Barbara