Shortly after Number of the Beast was published, he had a "Transient Ischemic Attack," which is, essentially, a precursor to a stroke. Looking back at that book, it's easy to see that he was having problems while writing it.
That might explain why he did reign in some on his last few novels (though Job was still "out there" at least it wasn't massive). Still though, I don't remeber hardly anything about "Friday" or "The Cat who Walks Through Walls" yet I can describe the whole plot of "Moon" and "Glory road" and ST and "Stranger" so even if he was more coherent, I don't think he was writing as well.
If he had stopped writing fiction in 1970 we wouldn't have lost anything significant except perhaps "The Notebooks of Lazarus Long"