Allow me to channel feminism for a moment:

Because having the old MEN hand down their business and equipment makes it seem like the WOMEN couldn't come up with the idea on their own, or invent their own equipment. Like there would never be a Ghostbusting business in that world if the MEN hadn't done it first.

/feminism

OK, it's me again. Anyway, Feig even said himself (either in the leaked emails or in an interview?) that he didn't know what to do with GB if it was set within the same universe, and would want the women to get the credit for starting the biz/building the equipment. So that's pretty much it. They have those cameos to show some half-assed support for the movie and thats about it.