My initial thought: figure out what constitutes a high value file, then target those. It is probably safe to state that virtually everyone is swamped in low value files, things like web browser caches, that any rate limit that doesn't affect the legitimate use of the computer would also allow ransomware to encrypt all of the user's data. As an example, there are currently 195 open files in my home directory. (Okay, this is Linux, but I suspect Windows is similar.) It is quite easy to trim my home directory down to about 2000 potential high value files with simple criteria I came up with off the top of my head, based on directory names or file types, without ever opening a file. Someone who knew more about the behavior of users and software could trim more.

Not only is the level of protection questionable, the possibility of breaking existing software is real, and file access on Windows seems to be slow to begin with.