But in my experience the PM is technical enough to understand what's going on. (Can write some SQL to answer questions, possibly ex-engineer themselves, etc.) They're in the same meetings, same email threads, looking at the same set of OKRs, etc. It's part of the engineers' jobs (B/F/D whatever) to communicate their constraints and their ideas (both product and pure-tech) to the PM, and it's part of the PM's job to take those into account when advocating for what should be done.
Similarly, the more the engineers know each others' specialties, the better they can coordinate. It's probably more important for everyone to have "a little bit of product" in them, but that doesn't mean we don't need a product-specialist.
When it's time for quarterly planning, the PM's voice is definitely loud, but they're still just one voice in the room. They're the one accountable for the product, which gives them some leverage, but the other voices are there (TLs, managers, etc.)
Now I can see this going terribly wrong if the scale is off (only one PM for too many engineers), or if communication breaks down (PM scribbles a "design" on a napkin and faxes it over), or if only the PM is consulted for planning. But the problem there is that communication broke down, not that it's bad to have a PM.
