A lot of prebuilt OEMs and consoles and not to mention laptops already use this approach.
On the other, my issue is that motherboards are already one of the least reliable components in your PC, and introducing even more complications to them just makes the situation (and the cost of replacement) worse
No that's silly, we should continue to use a design from 1995, itself replacing a design from 1981.
You can buy a nice AMD Ryzen motherboard for like $80 retail (Newegg shows $59 is where new unopened ones start). Normal electronics supply chain and shipping margins means it cost roughly half that to manufacture it. So on a $40 bill of materials and labor cost if you add even just $1 to make something better/modular/whatever then you're adding a significant amount in terms of percentage cost increase.
As much as reducing electronics waste is a worthy goal, the economics of it aren't aligned quite yet in today's world.