TL;DR I want a long lifetime of support for the chassis and more iterations thereof, at the same time.
I am not sure that the paradigm should revolve around the case being immutable whilst all the guts are replaced over time.
Because, when you consider it, the metal case is the easiest thing in the whole unit to recycle. Printer circuit boards and semiconductors, AFAIK, have to be made from virgin materials every time whereas aluminium can be smelted and reused infinitely. I see it as more important that the electronics should have the longest service life. The case remaining compatible is a means to that end, not because the case itself is the major investment of resources.
I refer to current threads requesting different features not in the current laptop such as deeper screen aspect ratio, and my own request for one able to accommodate a mechanical keyboard. Slightly different chassis might be needed to accomodate these things.
So imagine Jack, a video editor, and Jill, a screenwriter. Jack may want a more powerful computer to keep up with the latest processors, and the largest hard drive, whereas Jill values a good keyboard and that deeper screen to better fit the page onscreen. So Jill might buy the internals of Jacks machine to match with a case whose form factor is different and can accommodate a mechanical keyboard. Clearly they both have different needs and the laptop chassis might be optimised for both these use cases but the parts compatibility remains the defining factor. By making more iterations of case the electronics can enjoy a longer service life.
I think a better metaphor is that the laptop chassis is like a bicycle frame. Jack and Jill may both run racing bicycles on the weekend, and Jill may have a better tier of Shimano “groupset” which is the name for the mechanical (and often electronic or hydraulic) parts that make a bike into a bike rather than just a frame with wheels on. If Jill buys a new bike she might decide to sell Jack her old one so he can upgrade from his current “Tiagra” to the superior “Ultegra” groupset on Jills old bike. Jack would not, however, simply ride Jills bike as is, because it is probably the wrong size. This is possible because bike frames tend to have standardised interfaces meaning you can buy one knowing that your Shimano, Sram or Campagnolo groupset off another bike will fit.
As it is for bikes, I think the same for laptops. I think that Framework should look to become the Shimano or Campagnolo of laptops, and build their “groupset” of motherboards and expansion cards and all the rest of it, with the chassis being the bike frame. So more diversity of chassis please, since they only need to remain compatible with the parts already in circulation!