Using Rules to Prune Inactive Users - will this work?
Your solution "may" work, but another approach is to process the results of a VBO enabled view via Rules, whereas your rule performs 2 Rules Actions:


load the VBO list of users that are candidate for deletion.
add a loop to perform a Rules Action "delete user", for each user contained in your Views results.


For more details, have a look at the tutorial "How to process the results of a VBO enabled view via Rules?".

The advantage is that, if you first run it manually, you can see which users will get deleted (simply by browsing the Views results). When you feel comfortable enough, just automate this by rescheduling it every day (at the end of the rule itself, after you first triggered the rule manually).