Some people are griping that too much SS/TWOK made it into the film as is and you want them to have included even more, even at the expense of possibly losing screen-time for what was truly original in the film?
Not me. I already know the back story, am glad it didn't waste time going over his origins. That it contained so much original material despite being a rehash saves the day, IMO.
Are general audiences making a fuss over the vagueness of Khans origins? Not that I am aware of. Newbies wanting to find his origins can and will find their way to SS/TWOK eventually.