1. There's plenty of room to innovate on the implementation side. PMs don't tell you what algorithms to use or how to lay out the data in memory.

2. Not every engineer yearns to be a Product Manager. Sure, many want to make product decisions but don't want to do the other necessary work that comes with the role (like market research, metrics and analytics, managing the schedule and budget, writing PRDs, communicating with customers and partners, dealing with Legal, making sure the user manual is accurate, coordinating releases with ops and tech support, etc.) I think some techies think being a PM means merely sitting in your cube dreaming up a feature and saying "This Shall Be".

3. Good PMs most definitely need to stay on top of technology as it evolves. They need to know their platforms' capabilities and when new things become possible.