I don't think they did blow up the hero Y-Wings. I think they had at least 4 pyros built for exploding (3 blow'd up, and one left over to be refurbed for ESB). It doesn't make sense to blow up the heros as they have armatures and engine lighting inside. With all that work and prep going into them to make them good looking an functional, blowing it up seems ridiculous and the might be dangerous with the armament inside being turned into a missile. The pyro models were generally basically painted empty shells, with pre-scored sections to break away and the ones I've seen were suspended by monofilament and not on armatures. The triangles ship may not be around today, but that doesn't mean it was blow'd up. It could have just as easily ended up in private hands or was damaged during storage or moving.