I've wondered why there hasn't been a desktop with a CPU+RAM card that slots into a PCIe x32 slot (if such a thing could exist), or maybe dual x16 slots, and the motherboard could be a dumb backplane that only connected the other slots and distributed power, and probably be much smaller.

Those exist; they are used for risers ("vertical mount GPU brackets, for dual GPU" equivalent for servers, where they make the cards flat again).

PCIe x 32 actually exists, at least in the specification. I have never seen a picture of a part using it.

Retimers.

Sockets (and especially backplanes) are absolutely atrocious for signal integrity.

I guess if it's possible to have 30cm PCIe 5 riser cables, it should be possible to have a backplane with traces of similar length.

Cables much better sadly, so much so that they started to use cables to jump across the server main board in places.

VMEBus for the win! (now VPX...)

The hot stuff nowadays is µTCA: https://www.picmg.org/openstandards/microtca/

If I remember correctly the military / aerospace shy away from this spec because the connector with the pins is on the backplane, with the sockets on the cards.

So if you incorrectly insert a card and bend a pin you're in trouble.

VPX has the sockets on the backplane so avoids this issue, if you bend pins you just grab another card from spares.

This may have changed since I last looked at it.

Telecoms industry definitely seem to favour TCA though.

I don't know, I work in particle physics and here µTCA is all the rage nowadays.

Yes, for fucks sake, this is the only way forward. It gives us the ultimate freedom to do whatever we want in the future. Just make everything a card on the bus and quit with all this hierarchy nonsense.