I agree that it is hard for schools and the students to stay motivated if students essentially have offers in Jan of Yr6 and then have well over 2 years before they start their new school. So an exam might solve that, but it is toothless if it is an exam which doesn't have an impact onto the next stage. It's a tricky one. Senior schools which also have a linked Prep which allows entry for those Juniors without taking an entrance exam,also often struggle to motivate students and get them up to the level of those who were external candidates and had to prep to take an exam.
At the moment we are existing in a bit of an in-between system which is a type of limbo. CE served its purpose well in the days when many schools started at 13 and Preps ran to 13 and seniors were a bit more relaxed and happy to interview someone in Yr7 or 8 and offer based on CE results which wouldn't come until the summer of Yr8. CE provided a curriculum in line with the thinking of the time and was fact heavy and perhaps more like an O Level rather than GCSE approach. It kept Yr7s and 8s with their noses to the grindstone, and meant that when they started their Senior schools, they knew an awful lot, even if they weren't good at applying it. It worked.
But today, even for schools which start at 13, it's a bit of an irrelevance. Schools make offers based on pre tests in yr6, which are essentially 11+ deferred entry tests. More and more don't want to look at CE papers or use their staff time on marking them, nor think that their 13+ entrants will have spent 2 years cramming facts and learning inna way different to the modern teaching and learning approaches of the senior schools. Some 13+ Preps have broken away from CE and developed their own more modern curriculums in conjunction with the seniors they feed.......but people keep worrying that the students will lose motivation and laze around for over 2 years once they have a secure place....I suppose a bit like the concerns about Unis offering unconditional places and then A Level students being lazy and unmotivated and performing worse than if under exam pressure. It's a tricky one. Of course the yr7 and 8s already in senior schools manage to keep motivated and focused without end of yr8 external exams, but perhaps the teachers themselves also have more motive , knowing they will have to continue teaching them GCSE the following year.
It does seem that 13+ preps are almost but not quite obsolete and the CE really is pretty much already. In a system where few now transfer at 13 and even they are tested in Yr6, working out quite what yr7 and 8 in Prep schools should be like, how it should be tested if at all, and how to keep students and teachers focused on moving forward is really hard. And this difficulty in itself makes even more parents question whether remaining until 13 is worth it - when 11+ pulls at so many of them and the old 'benefits' and distinctiveness of CE are going. And the cycle continues with more parents leaving at 11 and the 13+ preps struggling and cutting back to 11.
I predict that within 10/15 years there will be a very small number of 13+ Preps. Most of the surviving ones will be boarding Preps focused on a small number of boarding schools which still start at 13and children will have to board to attend as most won't live near enough to one. Some of the senior schools will have moved to 11+ or predominantly 11+ entry and some of them will run junior schools which either go to 13, or in most cases 11 before transfer to the senior school. We will se if this comes about and if it does how long it takes to happen. The trend is already underway and set to continue as far as I can see. The 13+ prep will increasingly be a rarity.