McD's USA has never been a customer. If you look into the ownership model of McDonalds worldwide, you'll see that the corporate parent owns relatively few stores and does not dictate the information technologies that store owners (franchisee's) use. The country parent has more say in their country, than the Corp from my observation
Do people realise or know how 'sticky' these Apps are, and particularly the back-end platform technology (where the real magic happens)? Once they're installed and running marketing campaigns, getting the customer to change platforms is massively disruptive.
Not to say it can't or won't happen, it's just not as easy as you imply. I don't understand why now you think this undermines Plexure's credibility ... nothing new has happened with McD's USA ergo, you'd have to say that credibility in your eyes was undermined long long ago?
My guess is that if Plexure hypothetically secured all McD's worldwide, there'd be a whole new bunch of reasons to moan about them, like the chorus of "single client risk" which some say is already excessive.