Maybe the bottleneck for most high profile open source is PR review and not coding?
The most thorough review is done when you write the code, it takes much more effort to be as thorough when you read it as when you write it.
So if what you say is true then agentic coding will be net negative for same quality.
I've been by far the fastest engineer everywhere I've worked. I've produced 1000s of lines of code in a single day when I was really into it. I have nearly 500 Github repositories and almost all of them is a real open source application that does something useful. Think what you want though.
I certainly know engineers for which this is true but unfortunately they were never particularly thorough or fast to begin with.
I believe you can tell which way the wind is blowing by looking at open source.
Other than being flooded with PRs high profile projects have not seen a notable difference - certainly no accelerated enhancements. there has definitely been an explosion of new projects, though, most of dubious quality.
Spikes and research are definitely cheaper now.