You seem to be misinterpreting my thoughts as being against the idea of a product manager role. That's not what I mean. I'm talking about product as a separate autonomous org with a CPO, versus embedded PMs reporting to EMs alongside the rest of the product development team. I have similar, though less strong, feelings about Design as a separate org as well, for many of the same reasons (communication overhead/bottlenecks).
With regard to backlog, there is actually some capacity planning math that can show that (unintuitively) backlog size negatively affects product delivery velocity. I'm not an expert there, I've just watched some internal tech talks about it in the past.
At a higher level, think about what happens when the business side requests a new feature, or a product change. What is the difference in terms of confidence of estimates that can be given if you have 20 tasks in the backlog, versus 200. Sure, for that one thing you can up the priority and jam it in above a bunch of other stuff. But then, once that starts happening to people where you prioritize a newer request above theirs, how does that affect their confidence in your planning ability on a longer timeline?
> in my (vast) experience
Lastly, maybe pump the brakes a bit here, boss. This statement doesn't make me trust you more, independent of your ideas. We're on a web forum, not in a board room.