No idea how people come to the conclusion that it will be WW 1 or 2 just because of that teaser clip, it could be anything
But it most likely will be WW 1 or 2 era.
In terms of a marketing decision, it would be stupid to have "another" modern/future shooter as it won't stand out from the rest and as shown by the majority here, myself included, people are bored sick of modern warfare shooters now.
For marketing and sales purposes, it make the most sense to be WW 1 or 2 era, as if people want a AAA modern warfare or future shooter game, they can choose from countless COD titles, titanfall, battlefront, planetside 2 and even the other current BF games (BF 3, 4 and hardline) for that "fix" but... if people want a WW style shooter, what AAA game is there for them to choose from? Afaik, none so DICE/EA will have that market demographic all to themselves as people have nowhere else to go, which = lots of money.
Personally I think a future style shooter could be done quite well though, just because it is set in the future doesn't mean you need to have jet packs, lock on bullets etc.