I did some googling last night and various comments from those who had a hand in making the series and it was generally thought that it was intended to keep it going until one by one they died. Oh not what I wanted to read to be honest but Michael Apted said he hoped to be around to make 70up. Sad that things are taking a turn for the worse.
Michael was reported as saying "I hope to do 84 Up when I'll be 99". From a viewer point of view, it would have been fascinating to see it going until they all came to the end of their lives, but they and their families would probably prefer to retain some privacy in their older age and 'retirement' from the public eye. However, there was always going to be a flaw in that the film maker was obviously going to be significantly older than the children being filmed from 7yo - although sadly Lynn has already died.
Part of me thinks that it would be wonderful for it to be carried on by another (significantly younger) producer, but logically, it likely has met its natural end with Michael's death. The original (planned to be a one-off) documentary film was made by somebody else and Michael was a young researcher working with him (hence only being 15 years older than the children and not, as you'd expect, quite a bit more mature for a film maker in charge of the project).
In hindsight, it's a shame that Michael couldn't in turn also have taken on a new rookie apprentice of his own at, say, around the time of 42 UP, so that there would have been a younger person already on board, known well and trusted by the subjects and well-versed in the aims of the project, who could have just naturally and seamlessly taken over the reins after Michael's death (or declining health).
That's a wider part of the problem with some of the most fascinating history knowledge that we have - the whole point is that you have to wait a very long time before the whole story unfolds; in the same way as the most magnificent ancient oak trees will never be seen in their full glory by the person who planted them, or even their own great grandchildren.