It takes a lot of work to find, curate and distribute an entire course worth of material. As you said, open textbooks can provide some structure to a course, tweaked to the way the teacher would like to present the material. From there, the teacher can grow their library of resources year after year, until they have OERs that can replace a textbook.
In my consulting, I also do find teachers that have no interest in moving away from textbooks. Open textbooks might be a way to bridge the technology gap without leaving them (and their students) desperately behind.
I agree that open textbooks are a way to “bridge” the gaps (both in terms of technology and in terms of pedadogy).
I don’t really think that teacher have no interest in moving away from textbooks, but there certainly are a lot of challenges/barriers — policy, administration, parents/community, locked-down curriculum, technology availability.
I’ve seen a few cases where open textbooks have been used in a fairly traditional way (not that much different from proprietary ones), but that has then been a springboard to doing other things. That seems like a good leverage point.