I love doing analysis on Stray Kids' career, because their career is so interesting and unpredictable, so I think It will be big, sorry OP.
Maniac is bigger than Thunderous internationally and it is still doing over 420k streams daily on Spotify after 8 months. Its as big as God's Menu, especially If you take in consideration It had the same effect in their career on China that God's Menu impacted globally, both went viral with non-fans.
Case 143 has their second better first month on Spotify. I don't think It will be bigger than God's Menu or Maniac, but I think It has potential to be their third better performance overall there judging the numbers. Also, It's their biggest song on streaming platafoms in Japan and the third best by a male korean group this year - after Yet to Come and Hot - even If they don't have a bigger fandom there than a couple of groups, I'm seeing It will widen their audience on the country, so It's a win.
About being outstreamed by IVE and LSRF, they might be when It comes to title track, but Stray Kids has an advantage to be consumed when It comes to both their title track and b-sides keeping a good longevity and they chart in more countries (spotify, youtube or album, you choose) than groups with a more concentrate popularity in certain regions, that's why they are known for their popularity internationally.
About their mentioned domestic performance, Case 143 is their biggest and more stable song on korean plataforms. It's already bigger than any previous song by them on Circle Digital Chart and that had charted the longest. It didn't peak higher on Melon, but the stability is better.
They ate definitely not falling off, their consistence is actually impressive, even when they release title tracks that aren't as well liked as a previous one - what is normal for any act -, they are still on radar on streaming. There are many groups that If they release a less liked song, they will disappear from international stats due to fickle interest of casuals.
