"a very prominent and well-known design flaw" + "replaced on the spot regardless of age"
I don't think anyone on TPF has any objective data to back up these two claims. How many Portobellos have been sold and how many have been returned? Who knows, right? Could Portobellos show cracking more often than other bags? Maybe... but what is the likelihood that this particular SA has handled a Portobello return? Probably never.
Also, why is this a "design flaw" and not a "more delicate" bag?
*** Obviously I stand corrected - see OP's latest comments. ***