Acronym - You have a very narrow opinion of what kata is, how it's practiced, and what it's benefits are. I noticed that it was only well into the discussion that you attempted to limit your discussion to kata practice in traditional Okinawan karate and not the practice of kata and forms in the broader, general martial arts context. If you truly want to so limit the discussion, you should have 1. Posted in the Karate sub-forum, and 2. Stated that in the OP.
Not that you would have received any less blow-back.
Now as far as forms being mere instruction for beginners, I can't see that. In the Wing Chun lineage I've trained, we continue to introduce forms at many levels over many years throughout our training. And, although long past my prime, I still find new insights from practicing and teaching our most basic form, Siu N
im Tau, or the "Little Idea".
I will agree that forms, at least in WC are notheing like shadow boxing and have a very different function. On the other hand, the training sets in the escrima system I practice begin like kata ...each starting as a sequence of choreographed combinations performed against visualized attacks, and then become increasingly free-form in steps, sequence and execution as you advance, ending up being essentially very much like shadow-boxing.
Perhaps the take away here is not to make such over-generalized statements as the OP. Or at least if you do, understand that it will elicit a lot of disagreement and controversy. And maybe that's a good thing.