Again, it's a question of who your target audience is, and the practical matter of getting established with that audience. Dumas is still in print, yes, because there are still enough people appreciate classic literature and/or enjoy that writing style.
But a new writer flopping a Tome O' Wordyness out there is not dreadfully likely to catch up with him simply on the basis of "It's wordy, like Dumas.", assuming he's able to even get published simply because he's yet to establish much in the way of credentials with the classic lit fans. Most likely, publisher is going to look at and say "Yeah. Give me something shorter that might sell, then maybe we'll talk about your magnum opus here."
How many Dumas fans are going to buy a book simply on the basis that "It's long like Dumas, and has lots and lots of words?" They're going to need to be convinced it's worth the time to read, that that big old heaping pile of words actually add up to something worth their time. Unless the author has enough published material under his belt to make a say "hey, I'll give that a try."
Stephan Brust established himself as a successful and entertaining writer and built a reader base with the Taltos books before trotting out his Phoenix Guard. In all likelyhood, he was learning more and more about writing during that time period. In regard to the logistics of making a published book happen, without having laid that groundwork his odds of having the Phoenix Guard published would have been slim. On top of that, without already having led so many readers to Vlad's world it probably wouldn't have done as well. The publisher had a reason to trust that The Phoenix Guard was worth investing resources and money into, and there was an established base of readers who had reason to trust it was worth their time and 8 dollars.
Picasso's earlier work demonstrated that he spent time learning the fundamentals of depicting what he saw, and he then moved to a style where he depicted what he felt about what he saw. Having a foundation, a set of tools he was able to work with allowed him to do so more effectively than the current frustrated art students who never learn to draw but go straight to throwing fried monkey vomit at a canvas and then get frustrated when nobody understands how profound their work is. By mastering the basic tools, Picasso was able to create work that acted as a bridge from his earlier, realistic style to abstract style he developed over time.
It's a recurring theme, mastering the basics before moving to the more complex works. Michelangelo learned to draw simple things, to make pigments, basic composition before he did the Sistine Chapel, etc.
In the same way, Dumas most likely learned words and grammer and the fundamentals of communication before he laid trotted out The Three Musketeers. That's why most writers find it useful to start with short stories, character sketches, vignettes before they try to tackle a full blown novel. If a writer never learns to write a basic short story, never learns and exercises the skills to effectively describe basic events and emotions, it's not terribly likely they're going to produce the next Jitterbug Perfume, Ishmael or Atlas Shrugged.
