If those authors showed up today with those precise books using that exact language for the very first time, I don't think for a second that they'd be picked up by agents or publishers today.
What makes those stories timeless is that they have continued to be read and appreciated and reprinted in endless editions generation after generation. For example, The Secret Garden went into public domain in the USA about 1987. Publishers are still publishing it, screen writers and playwrights keep coming up with new versions, the book itself consistently ranks high in not only the best children's books ever published, but best books ever published. If that precise book using that exact language went around to book agents tomorrow, it's hard to believe not a single one of them would see it as something special.
The point being, they're not the timeless classics a lot of people make them out to be, things that will be eternally loved because they represent the height of perfection in prose. They're just books. They are a product of their time, just like everything else. A lot of it is only read today because it was forced on people when they were young and impressionable and didn't know any better. That's not to say they're bad, only that there are a lot of people who view them through convenient eyes, having b een told that they are good without actually understanding why.
I'd argue that some in fact do contain the height of perfection in prose. But as a complete product they may not match the preferences of today's readers or may just be too risky to publish. Only analogy I can think of right now is chess. The masters of 150-200 years ago would be destroyed by today's players, the style of most of the 19th century just won't work against players with a modern understanding. But that doesn't mean that the games or players of that era lack any brilliance (I'm using the common definition of 'brilliance' here). With a chess game though, you can appreciate a moment of brilliance without viewing the complete game but this is not how books are intended to be enjoyed, but I can see the argument that students shouldn't be forced to read the entirety of books not written for modern book readers.
The point with the classics, is that regardless of how good they are the gatekeepers in trad would not be publishing them today if they were not already established. Hence that self pub would be an option for works that time has shown to be quality. Regardless of the 'style' of the time or that those authors would not write like that now. Because some people do want to write like that, and those works do have merit, but they will have no chance in trad. Hence, quality works are set aside in favour of the money earners. And that's perfectly understandable. Self pub allows people who want to be different to be different, and still provides an avenue for that work to be released. hence, self-pub can have quality work. You just gotta wade through shit to find it.
not to mention that a lot of classics were 'self published' because the publishing industry didn't really exist at the time
Apart from Mill on the Floss and Under the Greenwood tree, which have some how persisted when they should have been killed with fire long ago (Even Shakespeare could have a bad day as anyone who's read Cymbeline can attest)
