It is certainly not the case that if you have not yet heard then you are borderline or likely to be rejected. I work at one of the 'slow' universities (not Durham) and in my department we are definitely still giving offers, in fact, we will probably make the majority of our offers in the next week or so.
The reality is that we are working with a system that was designed in the days when we offered BBC and could bang out offers by return to everyone who was predicted those grades or above. Except for a few marginal special cases, we considered nothing else.
Now our standard offer is A*AA, but we will give proper consideration to everyone with a prediction roughly at AAB or above. We have more applications, from applicants with more similar grades, for more courses, and we have more criteria beyond just predicted grades that we have to consider.
I don't know how my colleague is organising things this year, but based on previous experience the order we work through applications is something like: Decisions on 1) applicants coming through special programmes, 2) applicants we need to interview, 3) applicants to be considered for contextual offers based on formal criteria, 4) applicants flagged as being from target groups but who are not under consideration for contextual offers, (3/4 applicants for joint courses that are moving quicker than us), 5) applicants who are definitely not going to be accepted for our main degree but whose profile could make them suitable for our less popular degree, 6) definite rejections, 7) all others.
Even though the 'all others' group are the biggest group, being essentially comprised of good students from good schools, making decisions about the applicants in groups 1 to 5 takes up more time because we have to be very careful and very sure that they will be able to cope with the demands of the course.
Applicants from the 'all other' group ultimately receive around 70% of our offers and, aside from the special programmes group, they have the highest chance of receiving an offer. Sometimes we might skip ahead and make some offers to people from the 'all other' group, in part to get an idea of our likely acceptance rate, but a lot of our offers to people in that group come at the end of the cycle.
It's really, really not the case that we don't want those students or that they are borderline applicants who have a tiny chance of success. At some institutions, it may be the case that they are keeping borderline students waiting as reserves while they see how many of their prefered applicants accept or decline, but when you are talking about very desirable courses with very high conversion rates, like some of the ones mentioned in this thread, that is much less of a consideration.
I can't speak for Durham or any other institutions, but applicants still waiting to hear from us should definitely not give up hope. Their chances of acceptance are as high as they ever were.