Completely agree. The percentage depends a lot on both the age and scale of the project.
If a project has 100 active contributors, the mean contribution of new code will be 1% per contributor. How many of those are core? Maybe 10 or 20? So maybe there each writing 2-3% of new code and you have a decaying distribution? But that’s a moderately large project.
One of my projects started at 100% code written by me, but we did a release last year where none of the new code was written by me (it was all reviewed by me) and even the release was done by someone else, I just approved it. That’s a project with 2-3 moderately active contributors and a load of tiny drive-by contributors.