Re: "The root cause is now under investigation."
This ignores the fact that most of the RM's work is sorting and routing that post. Fewer items being sent means fewer lorries (and trains and planes) to transport it between sorting offices, less work to sort it to routes in the delivery offices, and less to carry for the posties. I'm not convinced that it's as much work to deliver 100 items to a street, as it is to deliver 200.
The number of letters have fallen over time since the '90s because people now use email instead of writing letters, and a lot of things like bank statements are electronic (although I still make mine send me paper ones). A lot of post is the same as it was twenty years ago, though; bills, begging letters from charities, birthday cards, more bills...
As for parcels? Well, except for the small light "packages", those aren't generally carried by the postie on his or her rounds. They get delivered by van, like every other courier. In most cases, though, RM are more expensive, which is why their volumes are dropping. They have relied on being the "default supplier" because people got into the habit of taking a parcel to the post office to get it sent. In today's world, the number of post offices has fallen drastically, and people are comfortable taking something to their local "parcel shop" to get it collected by
HermesEvri or UPS, or whoever, or paying 20p more (or however much) to have it picked up.