Brad — The framework that I’d use to apply usefulness of reposting is as follows:
1. Does it expose my writing to new readers over time ?
2. Does it spark conversation ?
3. Does what I’ve written get re-discovered by my former audience? Are they getting value from what I’ve posted again and again?
On the surface, the first question seems like it would be the most interesting one to answer. Is the list of your blog readers and Medium followers the same? If not, the advantages to have a brand new audience discover your writing for the first time might be well worth it. This would be a terrific, paid feature for Medium to provide writers, btw.
However, questions 2 and 3 are more nuanced and IMO, more interesting, since they deal with the question of engagement.
Is conversation happening on your older posts? As a reader of your blog, it does seem like posts on Feld Thoughts have more discussion and debate. Perhaps it is a characteristic of the audience on that site, which is highly engaged and has been with you for years. Or perhaps, if your Medium readership intersects with your Feld Thoughts blog readership, that might explain low engagement on older posts.
Finally, are your posts getting re-discovered by your existing follower base on Medium? i.e. do they re-read certain posts? These engagement stats are not published by Medium but it would be valuable (and not to mention, gratifying) to know this is the case.
So, this doesn’t really answer your question but instead poses a few ones.