In my old role, we used to build custom eLearning from the ground up. So we would plan and execute the design for many common devices such as desktops, iPads, surface pros, android phones, and every size of iPhone. This meant creating individual graphics and coding brand new modules for each size. You can imagine the amount of work and expense that went into making our modules responsive.
Responsive design allows for viewing the content across various devices. These days, if we know users will be accessing the content with their phones, we will opt to develop in Articulate Rise. Rise is excellent at responsive design because it automatically reorders content to be digestible, limits buttons, and mimics mobile design. Our learners are hospital staff who sometimes have to reference just-in-time (JIT) resources, which is best done from mobile. Considering learners' access to mobile is also an important feature to consider before designing, and is appropriate for our learners because the hospital supplies devices.
I think the future of designing for mobile devices also includes opportunity to utilize AR/VR technology. Because mobile technology is fairly new, I think we are just scratching the surface when it comes to learning application. It will take time to study the effects on learners, but in the meantime I think it is worth meeting the learner where they are, making learning accessible and easy, and exploring the possibilities.
References
Ivec, S. (2015, June 19. Desktop versus mobile learning. [Web log post] Retrieved from https://elearningindustry.com/desktop-versus-mobile-learning