What is considered to be a natural success to begin with?
The only title that got crucified for Jump pushing it was Toriko hence why its mostly seen as a joke (in terms of its mistaken mass appeal) nowadays because they went all out on a product they were never sure was going to be a huge hit to begin with. When you look into the top three manga of every decade on the TOC you'd see that they were always the top selling as well so why is it that all of a sudden when Nakana started that Dr. Stone and Neverland are pushed to the front when they former sales are no where near close to being good as the titles being pushed to the back? This was actually a very interesting subject Super Eyepeatch Wolf brought up in his video.
"What is considered to be a natural success to begin with?"
That's a good question. Does such a thing even exist? Is everything just "pushed"? What can be considered as not being pushed? Who knows for sure, right?

We should ask all of the WSJ EiC's, they're the ones that will give us the best answers.
About the sales, the whole manga market has had a big decline in sales. New series are struggling more and more to take off and we can definitely see this with WSJ as well. It's not just Jump that is struggling to get new manga that sell over 100k. Most magazines seem to struggle to even find something that can sell about 5k consistently, most of them are doing even worse in that regard. I feel like we'll see a lot of magazines disappear in the upcoming years as they'll struggle more and more to stay afloat.
Now, the print manga market is definitely in decline. We can see the statistics. Magazine sales are also declining rapidly. But, we are seeing a rise in digital sales. The question is, just how impactful are the digital sales? If someone buys a digital volume for a manga instead of a print one, does it make the publisher less money or more money? We'll probably never find out. When digital sales inevitably get bigger than print sales will that actually end up being a negative thing for the industry?
How many of those digital sales are just extremely popular manga which are also very successful in their print sales department, series with big established fanbases( The Kingdom's and the One Piece's). How many of those digital sales actually go into the newest manga that we see? I'd really love to know this.
We can still fairly easily see if a new manga will be cancelled if it has pretty bad print sales estimations and for the majority of the times the manga with the weak print sales do get axed. Digital sales clearly aren't doing much for these manga.
Why are sales declining? Multiple factors. The population in Japan is declining. Physical media is losing relevance in general as the digital age grows stronger. Now there's so much entertainment you can get access to using something as small as a smartphone. Maybe a lot of people that used to care about manga are now trying to focus on their careers and on establishing their own families. There's definitely even more to it. Most kids nowadays are more than satisfied with Youtube and nothing else and I feel like it's definitely the same with Japan. Jump has even acknowledged Youtube as one of their enemies.
SEW brought up a lot of subjects, but he didn't properly expand on them, that's all I have to say about that.