If we weight the importance of output and input, input would comprise with 98% of the efficacy, output would be 1% and more 1% to unknown factors.
It is not that you can or not become fluent from input only. It is that the best way to achieve it is to focus at least 98% of your effort on input.
980 hours of input and 20 hours of output have much better effect than 500 hours of input and 500 hours of output.
It's not a question of belief. It is a fact.
Yes, my numbers are skewed, but it is not possible to account it perfectly.
Anyway, probably 1% for speaking practice is already way too much.
For Japanese I've read far more than 30000 sentences.
Right now, only my sentences deck account for more than 3000 sentences.
Also, I've listened far more Japanese that I have read.
I'll take some lifetimes to do the same with output.