The airplane analogy is a good one. Ultimately, if it quacks like a duck and walks like a duck, does it really matter if it’s a real duck or an artificial one? Perhaps only if something tries to eat it, or another duck tries to mate with it. In most other contexts though it could be a valid replacement.
Just out of interest though, can you suggest some of these other contexts where you might want a valid replacement for a duck that looked like one, walked like one and quacked like one but was not one?
Decoy for duck hunting?
Are you suggesting LLMs are decoy for investor hunting?
In the same sly vein of humour, the first rule of Money Club is to never admit that the duck may be lame.