I doubt they would do 5% as only recently we were paying around 13%.
But yes, a seller fee would be better/easier for all.
But it isn't the only factor in this rollout, Simple Delivery is another and for many, myself included, is viewed as a breaking point.
I'm not going to let someone else decide how I pack and send my listing. It's just not going to happen.
So even if they do similar to your idea, unless they change Simple Delivery I still won't be selling and because I'm not selling, I won't be buying as much either.
From other posts I've read it seems like there may be more like me too.