Shonen Jump has a natural overly competitive and hard environment, also clearly don't make much of a distinction from newbies and veterans which is a problem per se but in good part is what made Jump what they are but not with the sacrificies that are the veterans, part of them don't even try to come back to the magazine anymore because why you would put your name in risk and be canceled like a nobody just because you're not doing so hot in the rankings? And most of the ones who try come back face early cancelation because like i said, is a overly competitive magazine where you will be treated like a newbie again and even if you actually sell pretty decently you will have no back up from the magazine if the "rankings" aren't good not matter if you made big money for them with your last hit.
Shonen Jump is what it is, they will hardly change but it's a long long time problem they have that the rest of the industry don't have because the differences with treatment and environment, and Jump Plus is making this even obvious lately with so many veterans going there and part of them finding success after failing to come back on Jump.
Just to note, i'm not saying series that sell 5k copies should just be there forever because the author's name, but a lot of the said "flop" from veterans on Jump aren't even flop at all, just that the Jump "standards" for keep something going on has the tendency of being a lot higher than the rest of the magazines, which makes certain cases looks much worse for veterans than what it really is.