This sounds much more likely, and would as you say, be a completely accurate statement.

Mail that can be digitized will eventually be digitized by its senders, whereas stuff that can't, well, that isn't changing.

I know in Australia the bulk of the mail I receive these days is ebay orders from overseas - my bills are digital, my statements are digital. The physical things that go in regular envelopes tend are stuff like new credit cards or registered mail like my passport of things.

So yeah, it does seem like an implicitly shrinking space.