Seconding a lot of this. Except that Duolingo’s Japanese course is terrible. LingoDeer is a good alternative. I can’t speak for Rosetta Stone, though.
As the proverb says, there is more than one way to skin a cat so yes, for some
people there is no need to study grammar to learn Japanese. However, I suspect for most people it will be quicker and easier to get proficient in Japanese by learning grammar. Certainly grammar guides like Genki and Tae Kim have been essential for me. Also, doing exercises is (at least for me) a bit boring but again very useful as it forces you to use your Japanese. Speaking and writing are very different skills to listening and reading and to be proficient you need to practice them all. Unless you live in Japan or have a Japanese partner you don’t naturally get practice on the former two. That is where grammar books with exercises can help.
A lot of my formative Japanese study came from anime actually, I watched an absurd amount and to this day on wanikani I am learning the kanji for words I learned through osmosis (on this level, 態と and 台詞 for example). I gradually absorbed a lot of grammar, and that has been good for having a general sense of how stuff works and understanding stuff that I’m learning now.
That said, there have been many points along the way now that I’m formally studying, that I have discovered that what I thought I knew was wrong, or confused. For example, I never knew the difference between an ichidan verb and a godan verb ending in る, or the way different verbs conjugated, or that an adverbial 形容詞 ending with く was not, in fact, a verb; many, many things that would never have tripped me up if I read a textbook. Now I’m having to “overwrite” my mistaken assumptions, of which there are many.
I think it’s helpful to use native material but diving straight in with no idea and no guideposts sounds like a recipe for redoing a lot of mistakes and reinforcing incorrect ideas which will ultimately delay your learning, rather than the intention of speeding it up. Imo.
I have yet to try LingoDeer really - maybe I’ll give it a go sometime! I’ve personally really enjoyed Rosetta Stone, though as with anything the activities can get repetitive, so mixing it up can be helpful. That’s partly why I like something like Duolingo, as a way to vary up my routine. Will be adding a lot more easy-ish readers to that routine as well, to continue trying to get as much comprehensible input as possible!