The PC-XT keyboard has excellent keyswitches but a lousy layout. The PC-AT keyboard is much better; the 122-key Model F terminal keyboard is the most modern layout you can get with capacitive buckling spring.
I've done that - put black M13 caps on an F122 - but what do you do about the Enter key on the numeric keypad? The F has a stabilizer wire which the M lacks, so if you put the black key on as-is it sits limply and doesn't click properly. Similarly, the spacebar stabilizer wire is different - how do you get the black spacebar to attach properly? I see that whoever did that mod changed the F to ANSI layout. I kept mine as ISO but that meant I had to stay with a few non-black keys.
Older Model M keyboards have a detachable cable. You can buy a replacement cable with USB adaptor built in, effectively turning it into a USB keyboard. For those with fixed cable, yes, the 'blue cube' adaptor works, or there are ones sold based on "Soarer's converter".
So... what is it you need that Mono doesn't do?
That's true but I don't think the required CPU power scales linearly with the display size.
The IBM T220 was launched in 2001 and has a resolution of 3840x2400, slightly more than 4k. Even back then, processing power wasn't really an issue, but getting enough display bandwidth was (four separate DVI cables were needed to get the full refresh rate of 41Hz).
Others have mentioned Topre keyboards; you might also like to look at Mattias quiet keyboards. But really if you are happy with the Logitech G15 then there is no need to change away from rubber domes - keyfeel is entirely a matter of personal preference.
Panasonic is about the only major PC maker which makes Apple kit look cheap. You pay a big premium for that extra ruggedness.
The real competition (in features, that is, not price) for an Apple tablet would be the Panasonic Toughpad 4k, a monster 20-inch tablet with 3840x2560 resolution (that is, 4:3 aspect ratio). It's a beautiful piece of kit but hugely expensive. Apple could put the same panel in a 20 inch "iPad Pro" or "MacPad" and if priced more keenly it could sell well among those doing graphics work who want something more portable than a desktop.
How do these banknotes fit into the rules of Rock Paper Scissors Spock Lizard?
You're right that Dell laptops are relatively easy to modify and upgrade - for a laptop. But still you can't expect to transplant a motherboard into any but the most closely related model. I upgraded my old M90 to an M6300 by replacing the motherboard, CPU and memory. For the M6400, I believe that the motherboard and case from the M6500 should be compatible (provided you change the CPU and CPU heatsink) but I cannot be entirely sure. The newer 17 inch Dell models have 1920x1080 screens instead of 1920x1200. You couldn't jam the older screen into them because it is physically a different size, even if the connector turns out to be the same.
I wonder whether installing Classic Shell would help users who are used to Windows XP and earlier versions.
As an English speaker learning Spanish I found it easiest to start with the word 'corrida'. Somehow the O vowel before helped to roll the RR. After that, 'corrida de burritos'. More difficult still, 'corrida de perritos'.
I think you're missing the point. It is not about hardware durability. The original hardware installed in 1974 has long since been replaced (probably several times over). It is the software that costs money over the long term - hiring programmers to maintain it. And it is the software that is the reason the system hasn't been replaced with something else.