I'll admit that while I've used the trash can but never taken it apart myself. But I can't imagine it would have been impossible to throw 2x Polaris 10 GPUs on the daughterboards in place of the FirePros.
For what is essentially a dead-end technology, I'm somewhat doubtful people would have bought it (since the second GPU is going to be idle and add to the cost massively).
the CPU being upgraded would have been much easier though I think.
I think on a technical level you're right, but you need to run two of them and they'd need a custom design like so:
https://i.ebayimg.com/images/g/RQIAAOSwxKFoTHe3/s-l1200.jpg
For what is essentially a dead-end technology, I'm somewhat doubtful people would have bought it (since the second GPU is going to be idle and add to the cost massively).
the CPU being upgraded would have been much easier though I think.