I see you haven't read NASA's aforelinked blog post where they explicitly detail the sorts of complexity it would add. Since you've indicated an unwillingness

I see you haven't read NASA's aforelinked blog post where they explicitly detail the sorts of complexity it would add. Since you've indicated an unwillingness to read the blog post I'll summarize briefly: A camera on the "light" side of JWST would need its own heat shielding and wouldn't show us any of the instruments. A camera on the "dark" side of JWST would need to function at temperatures close to absolute zero. It would somehow need to do this without disturbing the ultrasensitive instruments on the JWST. This would have been needed to be accomplished with, essentially, 10-30 year old technology as the designs for spacecraft like this need to be locked in far ahead of time due to the incredible complexity of deep space launches. In both cases the cameras would need power and data cable routing, and given the various harsh constraints involved (complexity, budget, liftoff weight, size) would have involved compromises in some other area.