The problem is that what people are expecting these characters to be is based on a narrow knowledge of the characters – Superman for example, it seemed most people who have a problem with Cavill is because he isn't Christopher Reeve!!! Whilst his Superman was incredible, it is a take on a character that has existed for almost 80yrs, and the Reeve portrayal reflects a small portion of that history – much the same as Adam West's take on Batman reflects a certain time, but should every Batman mirror his portrayal?
The Superman and Batman of the DCEU more accurately portray their comic book roots, however the vase majority of audiences do not know that and so just believe what they read in reviews and blogs.
MAYBE this is true for comic fans, but not many of the critics. Also, there are core elements of the characters that a "new take" can't change without making them different characters.
BvS was fundamentally flawed as a movie - plot, characterization and pacing were a mess. Luther alone made the movie near unwatchable. All IMO of course, but my point is that there were issues far beyond expecting a particular take on the characters.
To your point though, I would say if you are going to SUBSTANTIVELY change iconic characters (Batman killing people WITH GUNS, etc.) it begs the question of why you're using those characters. Mark Waid, for example, did a different take on Superman with Irredeemable that was extremely entertaining but by making it a superman parallel instead of superman himself, it worked better. Similarly, Elseworlds stories can be a great way to explore a different aspects of iconic heroes (Red Son, The Nail, etc.).
However, many of us comics nerds wanted to see the iconic version of beloved characters on screen (which Marvel is delivering relatively successfully).
Wonder Woman was a fantastic movie on every dimension on which MoS and BvS failed. (I somewhat enjoyed MoS, for the record, despite its many flaws - like Clark letting Pa Kent die - but came close to hating BvS for its idiotic plot, bad characterization and missed opportunities.)