[solipsism]

I agree with what you have to say about it being a plateau... but what are these companies where a senior engineer's impact is limited to the work they get done as an individual?

Part of the definition of "senior", everywhere I've read it, is that your impact has started to spread outside yourself and to the team level. If you're a "strong IC", you're not a senior, in my reckoning. You're a senior when you're pulling up those around you.