I find this sentence interesting and surprising. In education, they teach us how teachers always have to learn new ways and vary our teaching methods. They teach us how we have to always be evolving because a lot of elements changes over time (technology, approaches, values, etc.) That's why I find it surprising that the instructional design hasn't changed much!

I feel ilke this sentence resumes well what motivation represents in a distance education environment. I agree that it's not just individual and how it's a limitation for research.