I would agree that the consecutive number of times coming up with a breakshot/open table shot is the most impressive aspect of a 600+ run. I think that, if they wanted to try for a high run the way John did, by just focusing on that aspect of the game, there were several past greats in 14.1 who could have done that.
But that's not taking away from what he did- just that he had a motive and none of the past greats had a motive for even trying it on an ongoing basis. There are some modern day players who impress me as possibly being capable as well- again- if they chose to devote their pool life to that end.
As per someone running 100 balls as a goal- there are a lot of people who run one rack consistently, sometimes two- but can't figure out why stringing five to seven together seems so improbable. Besides some luck or a good roll vs. a bad roll - I look at three components of 14.1 that need to be reasonably mastered to run 100, once you can pocket balls consistently: 1. the ability to FOCUS on 100 shots without a miss - not really so easy as one would think. 2. the ability to read the table in setting up consecutive break shots - most 1 or 2 rack runners don't put enough practice into this aspect of the game, or just don't have the full knowledge, and 3. watch guys like Holhmann and Immonen- you really need to have great ability to move the cue ball to ideal locations on the last four shots, one has to be fortunate to get 3 or 4 stop shot positions to the break ball, rack after rack after rack, no matter how good you are at reading the table and playing patterns- it happens; but not very often for 7 racks in row.
Normally in a 100 ball run, one or more pattern and position plays to the break ball require professional level execution, not always, but on most 100 ball runs for sure.