AFAIK there is no mechanism for the difficulty of new blocks to fall. And if the only use is creating transactions between yourself and yourself, well you can also collect gravel.

> This doesn’t sound super useful, but it sounds about as useful as a bar of gold in a world where nobody considers gold to be valuable.

Even in a sub-industrial context and ignoring pretty much all of human history and assuming your gold has lost all of its extrinsic worth, your bar of gold could be molded or cast into plenty of useful things (light reflectors, heat shields, baubles, weights). Bitcoins, not so much.