But...they don't though?
I can see the point that they should have avoided either of the World Wars and maybe set it in the present day with Ares utilizing something like Rucka's SEAR Group that doesn't represent any particular real world thing. But to say the film provides closure to WWI with Ares's defeat is criticizing something that isn't really there, IMO. It's sort of creating a problem that isn't explicit in the film just to criticize it.