With Suicide Squad coming out and a new Batman trilogy rumored to be in the works, I think Deathstroke is eventually going to be cast, even if it is for just one movie. I would like to see the character in multiple films because he is one of my favorite DC characters and such a bad-ass.
That being said, I was reluctant with my fan cast at first because Stephen Lang is 63 years old. But at the same time it started making more sense to me to use an older actor. If Ron Perlman wants to do Hellboy 3 and Lang is in 5 times better shape than Perlman is, then why not?
CGI has come a long way in the last 15 years since George Lucas put Christopher Lee's head on a stunt double's body for Star Wars, and Deathstroke wears a mask as well so he can be covered up. Stephen Lang also keeps his body in peak physical condition, making people in the 20s and 30s look weak and small by comparison.
So at the end of the day I chose what I feel is the right fit for the character knowing it still take a little movie magic to keep him around for a while.