Thank you, HerNameIsLizzy. It only took me twenty years to acknowledge the idea, and another five to put it on paper. I sure hope if I get crazy enough and decide to tread over from the armchair of a reader to the stiff desk of a writer ever again, it wouldn't take this long. Or I will have to read it from my grave, lol.
On a more serious note, if books greater than 100K are not saleable, the collection of books I have between my husband and I must be plucked from the other world. I cannot even count in two hands in our collection that aren't over 120K, and 180K is "normal" in our eyes. We are not the rare breed, by the way. I have several favorites that are so large that they are divided into four or five (continuous) parts, with each part estimated to be about 180K ~ 200K. These are not the wisest to get into, unless you are ready to read 24 hours nonstop, like I did
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First, note that it's hard to estimate wc from print.
Second, most huge whacks of novels are from well-established authors.
The Stand is massive (and shouldn't be, imo); some of the later Harry Potter are massive (and shouldn't be, according to JKR), and yes, there are some epic fantasy that are large, and Infinite Jest, and etc.
For the 99.999% of authors (and in here I'm including most published authors), yeah, 400k words, or anything even close, is flat unsellable. It will not even be considered by an agent or house. The financial risk is too large.
For the .001% who can either do whatever the hell they please and have agents and houses at their beck and call (see above JKR, King, etc.), or the lottery-level rare person who finds an agent and a house who just falls in love with something 200k+,well, if you were King, you wouldn't be asking, and if you're not, stuff happens, but I'd no more bet on that than I'd bet on winning the actual lottery.
No one here is saying this stuff to discourage you, or to suggest that people you trust were blowing smoke.
We've all seen people with unrealistic ideas; we try to be realistic. No one has suggested your work itself is unpublishable - just that it's seriously not likely to be so in the trade market, in what was its 460k-word form.
Doesn't mean it can't be edited or split, or that it's not tightly written, but we can only go by our experience and make suggestions based on what we perceive to be the best ideas to help you reach your goals.