You're missing the point here. Ideally the 3rd gen kids will grow up to realize their 2nd gen parents made reasonable decisions even though they disagreed as kids, just as the cliché says (and like so many testimonials out there).

e.g. If gen 1 only used witty condescending remarks to teach their children that they're dumb, then 2nd gen can realize that this severely damages their kids' self-esteem. If gen 2 recobers from that abuse they may choose the path of taking their kids serioisly even with their shortcomings. The 3rd gen might take some kind of offense in that but there is less of a reason to grow in resentment towards gen 2 while they raise gen 3.

Take any family line. What generation are they at? Shouldn’t all bad parenting be weeded out by now?

People are not purely products of their parents behavior. You have not only genetic variation, but a lifetime of all the incredible complexity of life experience that happens outside the house and with all other humans. I could imagine these factors moving each person backwards and forwards along any scale of “progress” we chose to measure people by, generation by generation, at times overwhelming the positive or negative influence of their parents.

Maybe generation 3? One of my grandmothers worked on a farm in a developing country.

Times change. Subsistence farming doesn't cut it anymore. The lessons learned from that generation are something we have to grow out of.

This assumes that the kids themselves actually learn and grow at some point.

Some don't. It may be that the parents overcorrect and never teach their kids right/wrong, or it could just be the kid being a problem, or any number of other issues.

Well think of how bad things used to be, the progress is very incremental. Our short term perspectives as humans isn't good for seeing these patterns.

Also, there is no ‘right’, rather a series of attempts that fit on an ever shifting curve. There is no ‘ideal’ parenting except by sheer luck.

Doesn’t this assume a static target of ideal parenting?

Why would you think that parenting skills would improve in successive generations? Historically that doesn't seem to be the case.

And they learned a valuable lesson that it's okay/normal to terminate relationships with you.