You can run a low-power PC and check your email like it's 1980 just fine, and these days they're VERY cheap. A $30 5W Raspberry Pi is more than capable of running 'mail' on the command line. For a bit more power, quad-core 1.6ghz ARM SOCs are about $60-70. For fileserver type stuff, a 15W Liva with a Baytrail-M can be had for $115, or you can build out a full quad-core mini-ITX for something like $175.
The reduced power usage very quickly amortizes the cost of the hardware. If you would save 200W by retiring a dinosaur - that's $17.28 per month of 24/7 usage. If you've got a Rpi on for 4 hours per day checking your email - it amortizes out within 10 months.
This totally ignores the additional headaches associated with old, failing, unsupported hardware. You yourself described it as a "sinister amount of maintenance and stress".
You can make a philosophical argument here that a piece of hardware is never truly dead if it'll still push bits, but be honest here - if you're running up your power bill to avoid spending $100 on new hardware, if you're putting up with a PITA system for the heck of it - it's not useful, it's your hobby. Hobbies are fun, I think it's fun to keep old gear going too, but be realistic about the merits of such activities.
Entirely subject, utterly arbitrary. I can still use an 8-bit machine to write a good letter.
What do you mean by this? I struggle to understand the workings of a linear voltage regulator. To even try and suggest that any one person could "understand" anything much more complex is deluded.
How deep does your understanding need to go to allow you to use it? You'd have to go back 40 years to find computers that any one person has a hope of "understanding". Even at that point there was all sorts of abstractions - did the logic designer fully understand the power supply? Did they understand the CRT screen?
Computers do not die; their users do. ("s/Computers/ConsumerToy/g")&etc.
The leak can be plugged, and it starts like this: don't use technology you don't understand, or .. to put it likewise: use all technology with full understanding. The only reason we threw away our old computers was because we didn't think anyone could/should use them; they are nevertheless: still useful. The 'usefulness or not' is entirely a spiritual decision, however, because the machine does not care, and it is this fact - and ignorance of such spiritual components of so many facets of human culture in general - is the blind spot in the debate, imho. Things are only as they are, if we decide they are; the usefulness of a computing device is absolutely not dependent on its physics properties in any way other than: can you turn it on, does it work, can the user do something with it? It is the User who makes the decision, all along the way to actual, real, usefulness.
I'm still using my computer to send email, just like I did in the 80's and so on, albeit it now requires a much more sinister amount of maintenance, and general abstract stress it seems, emotionally, than the old hazeltine connected to a Vaxen we used to have to use to 'get things done'.
tl;dr: there is no fault in our machines but that which the user decides and identifies.