The reason I think shim6 is intrinsically much more deployable than 8+8is that it doesn't force *any* change on ULPs.
Sorry, I'm not buying this. Whether you change 1 module or 10, you're talking about a kernel swap. From a 'deployability' perspective, they are the same.
What has to be deployed is a new IPv6 stack. One host at a time. We know how to do that. It will take years, but it will happen, due to routine operating system upgrades. And there are no discontinuities of service - just progressive growth in the fraction of sessions that can benefit from shim6. Yes, it is a big change to the stack, but it's *only* the stack.
Changing the ULPs doesn't make this significantly harder. Yes, you have to have a careful transition, but this is not rocket science.
Tony