I suspect these particular areas in which you feel they are at deadweight may be missing much of their focus or you've experienced an organization that is misusing this role.
So here's the thing - I get the desired purpose of the role of centralizing GTM strategy. But in practice product managers are too far removed from the customer to be useful 80% of the time.
Basically I find that tactics that work for a given business should drive strategy more than vise-versa. This is a very unpopular opinion for sure.
In the end, messaging and positioning don't exist in a vacuum. The market responds to what it responds to. But because product marketers are too far removed from the stress of actually making messaging turn into revenue, they just don't have the skillset to be useful.
I wish product marketers could make positioning that actually works. But what happens is they make positioning that doesn't appeal to customers, and sales and demand gen pick up the slack and now have two jobs - to actually use messaging that gets results, and to make a middle manager happy with some data that helps them make useless reports upstream.
From the other side of the B2B marketing fence, I see demand gen burning loads of cash on "high converting", highly "tested" ads that look great in reports, except that those leads never turn into customers, because the PPC marketers never track their revenue that far, and those lowest common denominator leads aren't a great fit for many B2B products anyway.
I suspect that if your argument is "Well, they should build the product that those leads want" I can see why (a) that view would indeed be unpopular, and (b) why you would be in conflict with product marketers who want leads that match the positioning, not positioning that matches the leads.
When all you've got is a hammer...