SMS cannot be sent over wifi, true. However, with recent updates to WP, notifications can be pushed over bluetooth. I think that if a user is signed into the same MS account on both devices, and if the devices are linked over bluetooth and/or USB, all notifications should be pushed to Windows 10's new notification center. From there, MS should develop a Universal Messaging app that when used from a desktop, automatically syncs a sent message to your phone and uses the phone's data plan to send the SMS. No use of wifi there. This would definitely drive Windows users to WP because the allure of iOS+OS X messaging is strong, and Android (besides MightyText, sorta) does not have this capability.
Also since Messaging on WP is baked into the OS and not "just" and app, maybe they would bake it into the desktop OS as well since they supposed to be unified? That would likely make it speedier.