You are saying I should cron snapshot the cluster then keep the incremental between
Yes. This is standard backup procedure. Doing periodic full snapshots both gives you an upper bound on how many incrementals you have to apply (which, as you point out, can certainly contain information that is obsoleted later) and gives you extra redundancy in case of corruption of one of the incrementals (which otherwise becomes increasingly likely as time goes by).
I think with the feature I'm suggesting this wouldn't be necessary and IMO be less data to backup in the end
I don't think it's worth it. It would only be useful for the "restore to most recent possible time" and nothing earlier, because otherwise you have the "data mixed in from newer sstables in the compacted version" problem.
Additionally, at least one person has implemented map/reduce against snapshots, which is another point in favor of a "periodic full + incrementals" approach. (I'll go bug him again about contributing a patch, now that I remember it...)