At this time, there is plenty to watch on BR. This can be supplemented with hd from cable or NF streaming.
While there are thousands of shows not on hd yet that doesn't mean that that there are thousands that most are willing to watch...again.
I have not purchased an sd title in several months. The way new titles are popping on BD, most of my catalog collection will be replaced in 2 years. All new movie releases are done in hd.