Once US News started more heavily weighting 4-year completion rates, schools responded by attempting to select for folks who would complete in 4 years and providing more support for first and second year students. That left fewer spots for transfer into upper-level classes because of fewer dropouts / transfers-out.
Like I commented elsewhere, most students until the 60s went to Cornell to the Telluride house. That relationship was very helpful for generations of students. Deep springers still do follow this path, but much more rarely, i.e. every few years someone will go.
The stories from the modern era about a quarter or more of the class going to Harvard of UChicago are mostly gone now, although every so often an admissions director starts trying to get as many dsers as they can -- looking at you Columbia ;) But basically, top schools seem to accept about one deep springs student a year as part of their upper-level transfer class. For example, this year, harvard accepted just 14 students for upper-level transfer, including one deep springer.
So things seem to be getting harder, but the student experience here is so unique and the student quality so excellent, that they're still able gain admission to top schools despite the changing landscape (most of them get into one or more of these schools before they attend deep springs).
There has been some conversation as this has been taking place about new arrangements or additional support (e.g. 4 year scholarships, a formal relationship with another school, etc.), but at the moment, the need isn't acute enough, though vision for this aspect of the experience will be included in the next strategic plan.
Generally, the quality and curiosity of life on the ground here is what attracts great and weird students to enroll rather than a pipeline effect; applicants attached to that don't tend to make it into final class.