Gura achieving 3M subscribers is quite a feat on it's own but if you put into perspective how rampant her growth has been it becomes clear this might be a phenomena.
Here's a cool website I found graphing the sub count of the holomems. Gura obviously being that outlying blue line you see.
Some other interesting facts you can spot with this graph is Ayame raking up subscribers at the same speed
(or faster!) than her peers who are streaming
daily. Note that Ayame hasn't streamed in more than a month.
Here's the comparison between Ayame and Watame who streams daily. Also all the 5th gen who stream almost daily.
IHMO Ayame could easily compete with Pekora in terms of numbers and popularity if she wanted/was able. No idea about her health and the rumors about her having a more
busy private life. Until we get some evidence it's just rrats.
She's got incredible charm and even holomembers are dazed by her. It's something hard to quantify or describe but it's clearly what you need to become really popular.
Edit: Obviously all holos profit from fans that sub to all/most holo talent because they love the brand but here's Shion who stopped streaming, as well, in comparison.
the site:
https://trackholo.live/en/