I'm always suprised that she's the 2nd highest sub count, based on the amount of clips I see it seems like it should be Pekora, Suisei or Fubuki. But whatever she's doing it's clearly working for her.
I wonder if she'll ever overtake Gura, based on SocialBlade she'll overtake her near the end of next year but things can always change, especially since Gura's growth is slowed at the moment due to a lack of activity
Marine has a huge edge due to her MVs, she invests so much in them and the quality is ridiculously high, so they get a lot of views and popularity. Maribako even got picked up by local radios iirc.
That being said yeah, if Gura doesn’t start being very active again she’ll probably lose the top spot sooner or later (and I think it would be for the best, I’m guessing the burnout is also due to the pressure of being “the number one vtuber in the world”, she never liked the spotlight).