I understand your reasoning and of course I would love that both companies work closely together each one contributing what they know best for our entertainment. I've been dreaming about realistic virtual battlefields for years, but I still can't see where it would be the benefit to both companies that deserves such an epic endeavor. For some reason the film 'Lost in translation' comes to my mind - make it suntory time ;)
I think that a big chunk of DCS fans would purchase the respective ARMA/DCS updates but I'm not sure that this would happen in reverse. I do not think that most ARMA users would find it so attractive than DCS users. I mean, most of DCS users would/actually find comfortable firing an assault rifle in ARMA world, but I doubt that most of ARMA users would find amusing the complexity of a flight DCS module. At best, a niche of both niches would be interested. For the record, I've been enjoying BIS work since OPF days.
As a counterexample, DCS: P-51D module is a genius move, with a moderate effort they can potentially attract many customers outside the regular DCS users's profile.