Most good software out there were started by engineers and not by product managers and engineer only products are generally great and liked by people. Linux and the ecosystem is one example.
I am not sure what really happens after Product Managers come in that the product takes a dive. I have seen this repeat almost everywhere. You start with a small team of engineers and have a product that people love, now you need more engineers and hence you bring in a manager and managers almost always demand to have a product manager and the whole thing devolves into a game of promotion.
Doctors are lead by Senior Doctors, Lawyers are lead by Senior lawyers, whereas Engineers need handholding that also sucks the very joy out of engineering.
In my opinion, What could be more beneficial is to have Product Managers who also are senior engineers and who atleast do hands-on development 10% of their time. This is exactly how even the senior most doctors still perform surgeries.
unless it is customized B2B or B2G, where the engineers are not really interested in the mundane chores of the nitty gritty of a drop-down or having to have constant arguments on what the button should be called, you likely dont need a ProductMgr.
The other thing that happens as soon as productMgrs enter is engineers let go of the ownership of the product. The back button doesn't work? well good luck, that was not in the PMs requirements. now raise more unnecessary tickets and show the improved velocity.