Was it the studio or was it Ivan Reitman (director)? In every interview I've seen with Reitman over the years he has said he (not the studio) suggested to Aykroyd that the story was way too big and would be unfilmable due to the exorbitant cost, and he suggested making it a smaller, going-into-business story set in present day to make it feasible (and also to help with the comedy). He also suggested bringing in Harold Ramis to help Aykroyd with the re-write and to Aykroyd's credit he readily agreed. It also made it a no brainer to cast Ramis as a Ghostbuster (since Eddie Murphy turned it down to do Beverly Hills Cop instead). Ramis was used to writing for Bill Murray, who they were thinking of asking to replace John Belushi (who died after Aykroyd had written the first treatment), and Ramis and Murray were friends and had already worked together on screen successfully once before in Stripes.