This is not the enterprise world, I doubt if enterprises will pick up a phone that doesn't even support VPN after all this time.
The iPhone didn't get to its place by being the enterprise's #1 choice - it got there by being the people's choice. And people make the decisions at these companies as well. Even if all the big corporates do go WP, how much is that? 5-10 million, assuming of course that not all 2.1 million of Walmart's employees will get a phone...
Finally, you miss the biggest driver of BYOD - costs. Even with the varied platforms, any company will save money by just having their employees use their own devices (not considering instances where the company pays for the phone of the employee's choice, and that is anyway going to be 90% Apple or Android).