I bring up other magazines simply because no magazine is in a vacuum. While Jump mangaka feel like they're in a vacuum, as said by Kubo, their series themselves aren't. Yes, Slime was a popular WN and LN. That doesn't mean that will translate to manga sales, especially on this difference in magnitude. Shounen Sirius being a smaller magazine with fewer hits adds to how amazing it is for a Isekai story like Slime to be selling this much. My entire point is in reference to your point that an anime adaptation for a WSJ series is a "big deal".
Now in reference to your point about producers rushing to adapt WSJ manga "simply because its the biggest magazine", AssClass, BC and going back, Naruto and Bleach all had, relatively, late anime. They're not rushing because its the biggest magazine, they're rushing because those series have the most momentum at that moment and they want to capitalize on that. Its normal for producers to rush to adapt hot properties though they might not always get them out as fast as might like. This however ends up with cases like Heroaca which is arguably still running with the "shackles" brought on due to the impressions of the first season. AssClass is the biggest hit this decade and they went almost 3 years to get it on TV, getting it on air in January 2015. I've gone off point but producers are rushing to adapt hot properties and I agree that WSJ has lots of those but to say they're rushing to adapt because its the biggest magazine is not accurate.
I'm not saying Kimetsu won't get an anime but its just as big a deal for that series as it is for a series like Overlord.
I've gone on too much on this.
. Such an unlikely interesting premise with probably the weirdest, most tropey cast of characters that get better the more they interact with each other. Doesn’t hurt that every girl is thicc and looks good as hell, thanks Leoat. 
