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).