I think Manjaro will be here for a while to come. The distro is doing too well. Phil has a Manjaro hardware lineup so that’s usually a sign of good health.
Well, you can’t stop people from leaving your distro, but generally if you maintain an active and positive community, people won’t want to leave. Manjaro does this so I don’t think it’s in any danger. Their development team seems to get along well, so I don’t see a Mandrake level fallout anytime soon. Also, Sabayon is still around, they just have an irregular release cycle. I’ve played with it last year. It was fun. Mandrake’s demise sucked, but it still kind of exists in the form of Mageia and OpenMandriva.
The one distro I am extremely concerned about is Slackware. I think there’s only one person that maintains the entire distro. Still, it’s the distro that refuses to die, and is still very much active, I can’t help but wonder if the one person maintaining it would like some help. It’s been 4 years since the last release. They do have kernel 5.4 on their current branch though so that’s a good sign.