Jeff Blaine said:

…if you could illustrate the value in it.

The value is that your inbox is empty.

“Keep it empty” is a much stronger, clearer, and achievable motivation than “keep it under 5000 items.” A glance tells you if it’s empty or not.

If you don’t simultaneously commit to keeping the inbox empty (not by dumping stuff into a folder with a different name, but by handling the new stuff as it arrives), then you’re right - now you just have two big piles of junk.

“What it did do was ensure that the next day didn’t close with 5100 unprocessed emails, or that their week didn’t conclude with 6000 emails sitting there, etc.” No, it didn’t ensure that. The next day DID close with 5100 unprocessed emails: 5000 of which were placed in “DMZ” instead of left in “Inbox”.

If you have 100 more unprocessed e-mails at the end of the next day, you’ve failed to process 100 items. No filing system or technical trick will absolve you of the responsibility to .. um .. be responsible for your commitments.