So the purpose of the illegal-break rule is to prevent the game from being corrupted in one way or another by soft breaks.
In this just-concluded International 9-Ball event, I counted 99 illegal breaks in the 40 streamed matches. That was 1 illegal break out of every 7 breaks. Looking at just the breaks that were not dry or fouled, it was 1 illegal break out of every 9.
I don't recall a single one of those 99 breaks that I would classify as soft.
We have traded one type of "corruption" for another.
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Here's some more background information on this situation, using data from the streamed matches each year.
In the 2015 US Open 9-Ball event, the 3-point rule used the side pockets for measuring whether the break was legal, and illegal breaks were just 4% of the total.
In the 2016 US Open, the rule was changed to using the head string, and balls had to pass the head string (base of ball) to be counted. The result was that illegal breaks jumped enormously to 20% of all breaks.
In the 2017 US Open, the head string was still used for the determination, but a ball only needed to touch it to be counted. I.e., if the edge of a ball touched the vertical plane rising from the head string, it counted. The result was that illegal breaks were cut in half -- 10% of all breaks.
So, for 2017, the measuring line was essentially moved half a ball's width closer to the rack and the results changed a lot. And we had 3 different versions of the rule in the 3 successive years.
For the 2018 International 9-Ball, they used the same rule as for the 2017 US Open, and illegal breaks rose to 14%.