This actually sounds a bit like what happened with us when KB was sacked as coach. We went after the more proven coach in Allan Jeans while KB was still our coach and once we got him it was thanks KB good-bye. Freo's Board must have been wooing Lyon for a while to convince him and now they've got their man it was good-bye Harvey.
Smart move by Lyon if he wants to be a career coach even though he'll be called disloyal. He knows like everyone else does that the Saints are gone and are going to need a full rebuild. It would only damage his coaching reputation if he remained with St Kilda.