Realistically, they actually benefit from the rumors and speculation either way. It’s free marketing for the film as so many people are getting drawn into the hype and want to discuss their thoughts on the current theories about Doomsday making an appearance.
And that of course is good for the film in the long run, as it will ensure that it comes out to a lot of attention and a firmly focused look from the community. The presence of Doomsday is one of the first things that people will watch out for as soon as we start getting more official information about the film, but at this point we expect that the producers will keep this a secret for as long as they can manage.
They obviously can’t hide that forever though, and they will need to do some pre-release marketing before the film is actually out, which should also see the major revelation.
It might turn out that the new film is going to take the plot in an entirely different, unexpected direction, and while fans have been questioning the possible merits of a move like that (with many still wanting to see Doomsday first and foremost), we should keep in mind that the people running this production have a far better idea about what it needs than the majority of fans watching their progress from the sidelines.