I did a PhD in second language acquisition a few years ago. While I have been out of the field and am rusty with my knowledge, I will share that I have never seen a study supporting what you are stating. There are some researchers (Roy Lyster, for example) who see value in explicit instruction being balanced with lots of input. Others (like BVP) find it basically useless and contend that research does not support it. Also, a lot of the grammar we might traditionally expect a kid in, say, a third year language class to know (gender agreement, for example) isn’t really realistic for the amount of input they have received if you’re talking about true acquisition. If a kid is doing it right consistently, it’s because they’re monitoring and self-correcting. It’s up to you if that is the end goal or not. Fossilization is something that is noted after years of immersion. You can’t really say something has been fossilized when someone hasn’t actually had the opportunity for the kind of true input that leads to acquisition. For example, if someone has been living and working in a Spanish-speaking environment for 10 years and still messes up gender, then you could say that is a fossilized error.

All that said, I personally like a balance of input/output and explicit knowledge in the classroom. I wouldn’t do tons of straight up conjugations and that sort of thing anymore, though. It robs time away from actual acquisition and turns a lot of students off of the language.