First: we have different mechanisms to ensure that this issue doesn't arise. But putting up my en-user hat, I do understand your concern a bit.
The issue I see, is that building this in will make the solution not so end-user friendly anymore. You will need more target storage (in my case, your example would overrun my 1 TB USB disk (yes, I have a lot of data
)) and for most end-users, your retention is difficult to understand. End-users want something very simple (fire-and-forget)
Can you explain us in what situations you want to use this type of backup? Is this for servers? Corporate endpoints? What is going to be your target? And so on...
The better we understand your (and everyone else) use-case, the better we can prepare future versions.
All feedback is welcome