But we've had disk quotas before that mostly worked?

If anything it seems an easier problem than processor time.

I recall disk quotas on shared systems at university back in 1998 and I'm sure they existed before that.

Two thresholds IIRC, one at which you get a warning, second at which you can't write any further and the disk write operation fails.

I don't think they deleted files, it was just you couldn't write more than [quota] bytes to your disk.

Is there something particular about cloud based systems that prevent this from working?

ie. is this a specific problem with distributed storage?

edit:tone