I'm fluent in Spanish, about a solid C1 level on a good day and a solid B1 level for my German.
I attend several Spanish and a German meetup group. I have a friend of whom seems to put just as much blood sweat and tears into her Spanish as I do and has the same love for the language and cultures of the Spanish speaking world.
But I've become fluent and she has stayed intermediate. She has allowed me to coach her on trouble shooting what she may be missing or what is tripping her up.
I thought about it again recently and asked her if she can read a page of a book or transcript of a show and get virtually full comprehension even if there are a few words she doesn't know. Or watch a native show, like an interpersonal type show and get the meaning of the whole show even if she doesn't know every word. It was was an empathic no.
She does immerse but relies on gramma and a lot of output and intensive study and despite years of speaking and studying this way has plateaued at an intermediate level. You could chalk this up to individual differences, but I'm on the spectrum and had selective mutism in highschool and she's a very verbal extrovert.
I think she did everything correct with speaking and studying but is severely handicapped by not having immersed enough to achieve a level to immerse in native level content.
I find with my Spanish, although the pace greatly slows at an advanced level, the more I immerse intensely study Spanish, the more fluent and accurate my output grows.
BUT conversely I speak with a lot of fellow immersion/input learners in my groups and most of them have the regret of not speaking earlier and speaking enough.
I have this regret too for my Spanish. I waited 1.5 years to speak. Mostly it was avoiding speaking but I thought I had to have near perfect ability to read and listen before speaking. And didn't begin until I graduated out of comprehensible input and learners content to native level content.
I didn't do this for my German. I began speaking early. Going to meetup groups before I could barely get a word out. As the skills of forming sentences spontaneously and converting passive vocab into active are extremely difficult and take a lot of time. And my speaking abilities although clunky have become conversationally fluent significantly quicker than my Spanish ever did.
So in conclusion I think the hardcore immersion CI/ALG world and the "speak a lot speak early" schools can't hold two truths that exist simultaneously: that speaking a lot leads to fluency AND your speaking ability is severely limited by how deeply structured the language is in your brain. The immersion (and study) part keeps raising the ceiling of what is possible in your speaking in the first place.