The thing is that this implementation doesn't mean much in terms of cost, since mITX motherboards are already very expensive to begin with: in a $50 motherboard this would be problematic... but have you checked mITX prices latetly for high-end skus?
BUT, at the same time, it offers a huge functionality bonus since it streamlines the usage of different layouts that change the traditional orientation of gpu in relation to the motherboard without requiring shielded risers that are delicate, expensive, and a general pita in many regards (let alone space-consuming).
All in all this is a very very good idea, that I hope gets some traction.
This implementation would mean a brand new spec. New designs need to be... Designed and validated.
If high end boards are already expensive due to being mini itx, imagine a different sku that has a very specific use case (mostly only via sandwich designs).
Only two layouts benefit, sandwich and opposite side horizontal. Only sandwich case has any designs out in numbers.
The issue is that this spec won't replace standard mini itx. This means this will always be secondary to the standard mini itx and likely not get made in big enough numbers to offset the costs.
Standard and regular horizontal don't need a specific boards, node 202 and rvz01/02 series already employ a cheap pcb riser.
Mobo manufacturers are still gonna make mini itx boards, but adding a sku they can't copy and paste means it has to be built from the grounds up anyways. At some point, the end result is we should get at least a product in our hand?
Why spend $$$$ on an unproven design whose only features is for sandwich case essentially?
It is better to approach it holistically and provide something that benefits more use cases.
For small makers, in small quantities I feel also doesn't help them. If you want to use a slightly different design, mounting points that differs slightly, you'd have to design a pcb specific to your case to take advantage of this.
If the whole idea is to move to pcb risers, then it'd be tougher to spec a pcb for your case than it is just to select the riser from LiHeat.
And then now your customer are segmented between those who can buy an edge mini itx and those who can't or don't want to, and PCB or flex riser designs.