By losing coherence I meant that they lost sight of the original plot, and that the central themes were stretched and warped beyond the original vision to the point where the story loses any meaning.
Something like Hajime no Ippo. Sure the plot makes sense on a superficial level. But the past 700 chapters haven't contributed to the story in any way, shape, or form. Recent chapters have as much value as your average Rorschach inkblot.
(Not entirely relevant… but I wanted to use it somewhere.)
There are genres where this is a bit more acceptable. But the vast majority of those titles (that I've read) are long runners for the sake of being long runners. By the end of the story they're unintentional self-parodies. If you described the ending of Dragon Ball to a fan at the beginning of the story, they'd laugh at you.
From the series I've read on that list, it doesn't inspire confidence.