will Bitcoin Core always have to hardcode minimum relay feerates, responding to network / market shifts one release cycle after they happen?
There are a number of hardcoded things in Bitcoin Core that are based on “real world” values and updated every 6-12 months. The headerssync parameters depend on specs of modern processors (the speed a “Ryzen 5950X CPU thread can hash headers“), the length of the chain, and the release sunset date. Fallback fixed seeds hardcode a snapshot of current nodes on the network, which is highly ephemeral.
Values like network size and approximate computation/bandwidth costs change very gradually, certainly slower than the pace at which other parts of the software are modernized/fixed. The current proposal is to just be within an order of magnitude of what these back-of-the-envelope calculations come out to. I don’t think we should tweak it at every release, but it’s sensible and normal to have these values grounded in an economic reality that can change over time.
mitigate compact block relay issues related to minimum relay feerate
I completely agree with a more comprehensive solution to poor compact block reconstruction rates. In this case, however, we can diagnose the problem (at least the majority of it) as a single policy rule that is stricter on the network than for miners, and one that we should have revisited sooner than once every 10 years.