The assembly has advertised in the European Union's Official Journal for another architect or developer, either at a fixed price or in a 22-year leaseback scheme. The cost of development is estimated to be in excess of £20m.
Hyett said another architect could not finish the building to the same quality as Rogers. He said: "How can it be anything but a loss to have anyone but the original architect finish the project off? It should have been worked through. It was a stupid decision.
"Richard Rogers is an architect of the highest calibre with great experience. It seems to me appalling that the people of Wales and future generations should be delivered a project of this importance in what will inevitably be of compromised quality."
Hyett questioned whether it was ethical for an architect to take over a peer's designs. "We have a lot of talk about copyright, but what about 'copy wrong'?"
At least 12 architects and developers have expressed an interest in taking on the project, but the RIBA president's concerns have been shared by many firms. One-time Scottish parliament bidder Denton Corker Marshall is among the most recent practices to confirm that it is not interested in working on the assembly.
Hyett added that the problems in Cardiff were a result of teething problems in private-public procurement routes for public buildings.
He said: "Building procurement in the past 25 years has changed enormously.
"As we make the transition to new forms of procurement there are going to be difficulties – Cardiff is a symptom of that."
Hyett was neutral on whether traditional or modern procurement routes were more appropriate for public building works.
He did hint that it was the client who bore responsibility for the assembly debacle. He said: "The assembly has been unable to deliver a process to maintain an architect of Rogers' quality."
Hyett said he would not comment on the current discussions between the assembly and Rogers over the future of the troubled scheme.