It's a tricky one.
I think that most independent day schools and some boarding schools have moved to an 11+ intake as their main admission point. This has made it difficult for lots of Preps to keep going until 13 and lots have scrapped their Yr7 and 8 and just go to 11 now. They simply didn't have the numbers to run to 13.
Preps which have maintained their Yr 7 and 8 are either struggling for numbers in those years and likely to drop them soon, or are feeders for the big 13+ boarding schools and are able to keep going. It probably means that the intakes for those 13+ schools come from a smaller pool of Preps - either linked junior schools or the small remaining number of Preps with yr 7 and 8.
For the really academic schools, the pre-test selects the best candidates and the schools can feel confident that the boys will arrive into yr9 at a decent standard or bright enough to catch up, with or without CE. For the less academic boarding schools with a bigger number of less bright and motivated boys, there could be a real worry that without CE those boys might have coasted a bit through the years after the pre-test and really not be up to Yr9 level.
What the Preps need to be providing is the kind of education supplied in Yr7 and 8 in good independent secondary schools. These secondaries don't teach a CE style curriculum which is heavy in facts and regurgitation. They don't have just 1 Latin teacher and 1 specialist scientist and perhaps 1 Geographer who came to a Prep 30 years ago because they couldn't hack it with the older kids or in a big state class, but these secondaries often have large departments of 8-10 or more staff with very modern facilities and the latest techniques and a very hands on approach to learning. I do think it's hard for many Preps to replicate the experience of a large successful independent day school in yr7 and 8...... And of course in lots of ways they don't want to replicate it - they are specifically trying to offer something a bit different - an environment where children can stay younger for longer, where they can be big fish in smaller ponds rather than the little yr7s, where they can take on leadership etc too. But they also need to supply the academic rigour and teaching styles and experiences which schools expect children to have when they start in yr9 and really have to hit the ground running, because in many schools they are straight into GCSE.
Lots of Preps are developing new curriculums. They are working with the senior schools they feed to learn about exactly what they want in terms of knowledge and skills and what those schools are doing with their yr 7 and 8s (if they have them) and looking to adjust. For some it's easier - if they have lots of modern facilities and aren't a ramshackle old stately home with lots of extensions built over the years, and if they have a flexible staff. Some though do find it difficult because CE is what they've always done and the staff are a bit set in their ways. Probably there are less and less schools like that now in reality.
So overall, I think that 13+ entry will continue to dwindle...not to nothing, because the big boarding schools will keep it going, but many other schools will take more and more at 11 instead of 13. The number of Preps going to 13 will continue to drop and a special curriculum for Prep school yr7-8s will be need for fewer students. With boarding fees high and pre-testing happening in Yr6, more parents might ask if their children are being tested in yr6 and the places available for yr7, why not just take the place then rather than delay until yr9, especially if they are not dead set on boarding.
It's definitely a changing market.