It has nothing to do with Becky leaving but everything to do with keeping the story line continuing. (If that's the route they're going.) Ronda is already getting booed, might as well make her go full Heel. What better way to do that than align herself with Stephanie? Ronda will eventually have to become Heel since none of the other Four MMA Horsewomen are Faces.
Another option is include Charlotte as the McMahon's chosen one. It saves Ronda from having to turn Heel and will take the heat of Becky losing.
Recycled men's storylines is arguably better than anything they've come up with the women for awhile. Between Alexa/Nia and Mandy/Naomi, they're not that enjoyable.
They should have just continued the same story telling as when Becky was attacking Charlotte during their feud. Photoshoots, person in the crowd. Anything to put herself over rather than relying on others to do it for her.