In my experience (as a hiker, not a mountaineer), perfect knowledge is never to be had. One does the best that one can, based on weather reports, trip reports, and one's experience.
To wit: Last weekend, I did a quickie loop from the tram. It was a tad icy. I opted to ascend the Sid Davis route and descend via the main Round Valley trail. The Sid Davis route follows the north side of a small creek and therefore has southern exposure. My reasoning was that because of said southern exposure, the snow would be softer going up the Sid Davis and then as the day progressed, the snow would soften on the main trail. I assumed, based on past experience, that a route would be well tramped down from Tamarack Valley (where the Sid Davis route goes) to Round Valley (where the main trail goes). I was wrong. I found no evidence of a tramped route. However, I had brought a topo map, a compass, and a GPS. Sufficiently prepared, I had no difficulty making the traverse from Tamarack Valley to Round Valley although I did ascend 100 vertical feet too high at one point on the traverse when I tried to follow some tracks (instead of breaking trail) hoping they would lead to Round Valley. We did have a nice lunch spot to ourselves though.
As we descended the main trail, I found that I had been correct; the trail had softened during the day -- just as it normally does.
But what if the day doesn't go as expected? What if instead of softening, the trail had iced over? I think that's what I'm reading about Ellen and Sally's case -- and I think that the snow was not only icy but unreliable too (in sense that it wouldn't hold them; the surface would break off). Were I in a situation where a) the conditions changed in a manner I did not anticipate and b) the snow was unexpectedly unsafe to descend, I also might find myself stranded and in need of a bail out.
For the future, were it I, I might check my elevation progress more regularly. Maybe then one could decide to turn back earlier? Maybe. Dunno. Again, changing conditions cannot always be anticipated. By the time one realizes that one cannot descend the way one came (even though they had thought with good reason that they could), one may already be fairly far along on a route and have few options.