We wish there was a standard too! We designed the mainboard in the Framework Laptop to be replaceable and upgradeable to future generations, along with the upgradeability of memory and storage. We've be developing new mainboards ourselves, and we'd love to bring in third party compatible mainboards too.

I would love to make an arm board. Would be cool if you could publish some sort of reference design EDA files with just the board outline and IO connectors populated.

If this goes anywhere let me know - I work on BSPs at Arm

That is doable!

All the sub-3lbs ultrabooks I've owned so far follow a similar design, the bottom of the laptop acts like a tray with the top half taken by the motherboard/heatpipe assembly and the bottom half being filled with battery.

There are differences in where the display/USB ribbon connectors are and speakers are always in strange places. Sometimes the bottom tray is tapered off like a MacBook Air. Some ultrabooks are 9mm thick and others are 11mm thick, but at that point, honestly who cares?

But in general, I don't see any reason why a 13" standard for laptop motherboards+heatpipe can't exist. Out of the XPS, HP, and Asus systems I've upgraded in the last 6ish years, they've all looked more or less the same but with annoying 1-2cm differences in ribbon connectors and screw placements.

If anything, standardization might lead to more efficient use of battery compartments. A lot of laptops seem to have empty air surrounding a commodity battery that could have been 55Wh instead of 42Wh if the space was used better.

laptops with discrete GPUs usually have an integrated heatpipe solution that cools the CPU, GPU, and chipset. at least that's been the case with all the ones I've opened up. so you would probably have to replace the GPU also. even without the GPU, the CPU/RAM/mobo is already a pretty big portion of the BOM.

at that point, I have to wonder how much you would actually save by not replacing the screen, chassis, and storage. overall, this concept strikes me as more of a win for repairs than upgrades.

Many popular 'standards' needed someone like an Intel or an IBM to push it into the mainstream by adopting it and making it very popular. For design standards on the physical layout of components, individual component manufacturers stand to benefit the most because they can sell upgrades. The system integrator (HP/Dell/Etc) is unlikely to push for them as they want to just sell you a brand new device.