as long as SHA-3 ASICs don't exist and SHA-256 ASICs do, no one will use SHA-3. So there would be a softer transition from SHA-256 to SHA-3,
There would be no difference between this and simply making a cut off point at some point down the road, IMO, as you say yourself no one will use SHA3. It is pointless and adds unnecessary complexity.
I do think there would be a difference. Developing SHA-3 ASICs and targeting the release date to exactly when the cut off point occurs is pretty damn difficult. If people were told that SHA-3 can now be used, businesses would slowly start developing SHA-3 ASICs, and they would be able to put them to use as soon as they finish development.
