... which is great until a consumer or replica decides to replay a topic/partition from the beginning, and forces all the recently-written data to be evicted, adversely impacting performance for all the well-behaved consumers :(

If you're running a Redis instance as the only thing on the VM instance it's on (very likely in my experience), is there a reason to reimplement all this policy logic in Redis, rather than simply tuning the caching policy of the existing OS kernel to make it perform well for Redis (maybe at the expense of performing well for any other process you might run under that same kernel)?

Certainly we can do smarter things if we know an access is going to result in a page. But we're not in a position to take advantage of that until green threads are in place.