You are absolutely right. After all, I suppose, if there was a method that could be taught for writing "memorable" tunes every composer would be able to write them. But therein lies inspiration. Elgar (who was quite good at writing tunes) said of one of his greatest (the Pomp & Circumstance March No. 1) that the big tune came into his head when he was out trout fishing!
No, not really. Many composers hate the very concept of a theme. Few composers would want to write this way because it is "quaint" and not a style in vogue. George Lloyd's slow acceptance is proof of that.