As a lifelong Python fanatic, I always thought that (other than Gilliam, who rarely "acted") Jones was the most limited in terms of the types of roles he played. He excelled at playing middle-aged pepperpots (especially in Life of Brian), and the typical straight man "city gent" type, but he didn't have the versatility of the others. His true strengths were in his writing and in directing. He and Palin were the ones who wrote most of the visually interesting Python skits (the ones shot outdoors), and his direction in Life of Brian was absolutely superb.
I always loved reading Cleese's memories of the endless writing room battles he used to have with Jones.
I, too, thought that Jones would be the next to go. Back in 2014, when they were doing the live Python reunion shows you could tell that Jones was struggling to remember his lines and that the others had to ad lib around him when he faltered.
RIP.