The idea I am trying to work out is: Without literally duplicating a seed / HWW and stashing them - what other "failsafe modes" do we have to retrieve our Bitcoin just in case we lose access to either the seed or device.

The risk with having extra copies of a seed hidden in different places is that they all become theft risks and each seed give "full permission to spend" (unless you have a passphrase, which can be brute forced or forgotten, and introduces its own management complexity etc). Whereas, by using a pre-signed unbroadcast transaction to a fallback address you have something at least, that you can can use in an emergency to move the funds if all else fails.

Even with a passphrase in place, if your HWW screen fails, and it just so happens that the place you thought your seed was stashed has been compromised, suddenly you have no Bitcoin anymore. In most people's setups I imagine their whole setup could silently fail and they might not even notice for months.

Some of the comments in this thread don't seem to grasp the value of this, which makes me wonder if I have overlooked something.