So adapting the wording of the current disk space purge option, you want a setting that says, "Purge the oldest set(s) if the total size of all backups exceeds XX GB on the target volume". In this case, "all backups" would be defined based on whether you've selected "matching backups" or "all backups in the destination folder" in the dropdown above your retention policy settings. I guess I see some value there, but if I had to guess why it doesn't already exist, it would probably be that storage is so cheap these days, and backups are typically so important, that most people would consider how many backups they want to keep and then make sure they have enough storage to retain that number of backups, rather than shrugging and saying, "I'll take whatever I can get with X amount of storage." Retention policy should determine storage capacity, not the other way around.
Or is your thinking basically, "I expect my allocated storage will be enough for what I actually need
, but as long as the backups stay within that limit, I want to let them continue accumulating just because"? In that case, depending on your schedule, if you had this option it would probably make the most sense to just disable the Full retention policy entirely so that backup set growth would be completely unrestricted except by this total size limit. It might also make sense to disable the other retention policies as well. In the latter case, the catch is that you'd only ever purge by entire sets rather than individual backups, but with frequent Fulls that wouldn't necessarily be a problem. In fact, you'd want to have fairly frequent Fulls anyway to make sure that a "set purge" triggered by this setting or the existing disk space purge mechanism wouldn't wipe out too many backups.
In my own case, I've always just retained what I needed, and if I don't need it, then I don't retain it. So I set my retention policy according to my needs, and if I underestimated the storage requirement, then I add more storage. If I instead have storage left over, then I don't feel any need to use it for backups that I've already decided I don't need. Maybe it's my "anti-clutter" mentality, which itself may be the result of being the son of a collector.
Anyhow, this should probably be created in the Wish List section, even if the thread you create there just contains a link to this thread.