If that's the case, why even do it in the first place? It's not like they didn't know that they were going to use Batman in their DC movie universe. I guess my point is why even decide to position Batman at the "end of his run" if they aren't going to do anything with that in the movies. You can make Batman older and more experienced with out explicitly placing him near the end of his career. Again, I ultimately doubt they do anything with that idea (which I think is a bad one at this point), which makes it all the stranger for the filmmakers to bring up to begin with. It just reiterates my belief that Snyder has only read The Dark Knight Returns, and is basing a lot of this off of that.
The point is it offers an older and more experienced Batman (at least in comparison to Superman), and I'd suspect Batman will play a bit of a mentor role to Kal-El as he develops his Superman persona in future films (Justice League, maybe a cameo in Man of Steel 2, etc).
Plus, we've seen the Batman origin countless times in comics, films, TV, video games, etc...
We haven't seen an older and more seasoned Batman on film yet (well, arguably Bale in the third Nolan film, but he was in his early thirties max). It offers a lot of storytelling potential. It also offers the ability for them to do prequel tales of Batman earlier in his career, to flesh out the DC film universe mythos, should they choose to do so.
It's a new approach for the multiple film versions of Batman in that regard.
It also allows him to automatically assume a leadership role in the JL due to his experience (though Wonder Woman is arguably more experienced in battle, depending on how they approach her origin in the films... is she hundreds or thousands of years old in this version?).
Also, here's an interview Zach Snyder did with EW.com, explaining the decision to destroy half of Metropolis. In it, Snyder says all the dumb things that you would expect, such as; "I was surprised because that's the thesis of Superman for me, that you can't just have superheroes knock around and have there be no consequences," says Snyder. Maybe it's just me, but I've never really thought that this was the thesis of Superman.
http://www.ew.com/article/2015/07/02/man-steel-ending...-zack-snyder-makes-his-case
I think it's just a poor choice of words. "Thesis" absolutely doesn't apply.
The better phrasing would've been that "such super-powered battles have large and horrifying results", which is the real world result of such things. In fact it's fantasy almost to a level of irresponsibility to think such battles between Superman and his foes don't regularly cause pretty massive destruction in the comics, or the Hulk, or any such uber-strong character.
Their fisticuffs will have massive ramifications for the human world around them.
Hopefully part of Clark/Superman's evolution as a hero and character in general in the future films will show him focusing on protecting lives (admittedly one of my biggest gripes with Man of Steel) and becoming the far more capable (and strategically-minded) character portrayed in the comics and Reeves films (the first two anyway).
In fact I'm positive Batman v Superman will cover that very topic, with Batman accusing Superman of being irresponsible and making rookie mistakes that cost the lives of tens of thousands. Hence Batman's application in being a necessary mentor role for Superman as he learns to develop his powers, etc. Minimizing casualties and collateral damage, thinking strategically, etc will hopefully all play into that.
I kind of like the idea of Batman as a mentor to a younger Superman.