Why would it need to be thicker (and less appealing)? It is already possible to fit motherboards of different designs. There may not yet be a profusion of alternative motherboard options available, but they are possible, and some do exist - for example:
I mean for reducing e-waste, repurposing old motherboards in a laptop equivalent of a pc tower. Not so much brabd new motherboard alternatives you can put in it. I thought the point of repairability and user servicing was to reduce e-waste by making your device last longer. It just seems strange to me.